Default connection pool size of RestClient is too small.

[ujjboy]

Describe the bug

See code at com.alipay.sofa.rpc.transport.rest.RestClientTransport#buildProxy

 ResteasyClient client = builder.registerProvider().logProviders()
            .connectionPoolSize(transportConfig.getConnectionNum())
            .build();

The default value of transportConfig.getConnectionNum() is 1, the performance of rest client is dependence to connection pool size.

Steps to reproduce

Use rest client with multiple threads.

Environment

• SOFARPC version: 5.3.2
• JVM version (e.g. java -version): 1.7.0_80
• OS version (e.g. uname -a): OSX
• Maven version: 3.2.5