Public API does not work on WildFly

[felixbarny]

See https://discuss.elastic.co/t/java-apm-agent-throws-classnotfoundexception-in-wildfly/158779

Reproduced in #361

[eyalkoren]

Considering all factors, such as

• implications on users
• safety
• robustness
• maintainability

I chose to go with the big refactor (@felixbarny second suggested option), so that all agent code is rooted in the co.elastic.apm.agent package. OpenTracing and public APIs are not under this root.
The org.osgi.framework.bootdelegation setting was updated to allow delegation only for agent-specific classes. In addition, the actual fix for WildFly was through same enablement using s
the jboss.modules.system.pkgs setting.

I moved the changes from #361 that reproduce the problem to the refactor+fix PR (#376) because this breaks the build, so closing #361.