Avoid spamming logs with errors about SR if SR not configured

[big-andy-coates]

It is not required to use KSQL with the Schema Registry. Yet if it is not configured the logs get spammed with lines such as:

[2020-01-15 12:19:08,701] WARN Could not clean up the schema registry for query: _confluent-ksql-default_transient_862477730803062421_1579090748245 (io.confluent.ksql.schema.registry.SchemaRegistryUtil:60)
java.net.ConnectException: Connection refused (Connection refused)
	at java.net.PlainSocketImpl.socketConnect(Native Method)
	at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
	at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
	at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
	at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
	at java.net.Socket.connect(Socket.java:589)
	at sun.net.NetworkClient.doConnect(NetworkClient.java:175)
	at sun.net.www.http.HttpClient.openServer(HttpClient.java:463)
	at sun.net.www.http.HttpClient.openServer(HttpClient.java:558)
	at sun.net.www.http.HttpClient.<init>(HttpClient.java:242)
	at sun.net.www.http.HttpClient.New(HttpClient.java:339)
	at sun.net.www.http.HttpClient.New(HttpClient.java:357)
	at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1220)
	at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1156)
	at sun.net.www.protocol.http.HttpURLConnection$6.run(HttpURLConnection.java:1040)
	at sun.net.www.protocol.http.HttpURLConnection$6.run(HttpURLConnection.java:1038)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessController.doPrivilegedWithCombiner(AccessController.java:782)
	at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1037)
	at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:984)
	at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1564)
	at sun.net.www.protocol.http.HttpURLConnection.access$200(HttpURLConnection.java:91)
	at sun.net.www.protocol.http.HttpURLConnection$9.run(HttpURLConnection.java:1484)
	at sun.net.www.protocol.http.HttpURLConnection$9.run(HttpURLConnection.java:1482)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessController.doPrivilegedWithCombiner(AccessController.java:782)
	at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1481)
	at java.net.HttpURLConnection.getResponseCode(HttpURLConnection.java:480)
	at io.confluent.kafka.schemaregistry.client.rest.RestService.sendHttpRequest(RestService.java:253)
	at io.confluent.kafka.schemaregistry.client.rest.RestService.httpRequest(RestService.java:325)
	at io.confluent.kafka.schemaregistry.client.rest.RestService.getAllSubjects(RestService.java:724)
	at io.confluent.kafka.schemaregistry.client.rest.RestService.getAllSubjects(RestService.java:719)
	at io.confluent.kafka.schemaregistry.client.CachedSchemaRegistryClient.getAllSubjects(CachedSchemaRegistryClient.java:457)
	at io.confluent.ksql.schema.registry.SchemaRegistryUtil.getSubjectNames(SchemaRegistryUtil.java:58)
	at io.confluent.ksql.schema.registry.SchemaRegistryUtil.getInternalSubjectNames(SchemaRegistryUtil.java:75)
	at io.confluent.ksql.schema.registry.SchemaRegistryUtil.cleanUpInternalTopicAvroSchemas(SchemaRegistryUtil.java:45)
	at io.confluent.ksql.engine.KsqlEngine.unregisterQuery(KsqlEngine.java:253)
	at io.confluent.ksql.engine.EngineContext.unregisterQuery(EngineContext.java:228)
	at io.confluent.ksql.util.QueryMetadata.close(QueryMetadata.java:155)
	at io.confluent.ksql.util.TransientQueryMetadata.close(TransientQueryMetadata.java:108)
	at io.confluent.ksql.rest.server.resources.streaming.PushQueryPublisher$PushQuerySubscription.close(PushQueryPublisher.java:105)
	at io.confluent.ksql.rest.server.resources.streaming.PollingSubscription.lambda$request$1(PollingSubscription.java:86)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at com.google.common.util.concurrent.TrustedListenableFutureTask$TrustedFutureInterruptibleTask.runInterruptibly(TrustedListenableFutureTask.java:125)
	at com.google.common.util.concurrent.InterruptibleTask.run(InterruptibleTask.java:57)
	at com.google.common.util.concurrent.TrustedListenableFutureTask.run(TrustedListenableFutureTask.java:78)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)

Whenever a query is closed.

KsqlConfig.DEFAULT_SCHEMA_REGISTRY_URL probably has something to do with it. We should't have a default! (Though when we remove it we should ensure KSQL is still hooked up to CP's SR by default).

Instead, is no SR url is provided we should assume its disabled. So we shouldn't try to clean up this stuff.

Probably the cleanest, least invasive, way of implementing this would be to provide an alternative impl of SchemaRegisteryClient. One that returns an empty list from getAllSubjects (so we don't try to clean anything up), and throws on any 'put' style operation.