Remove freeInboundBuffer() and freeOutboundBuffer() which has no value

- Fixes #1308

freeInboundBuffer() and freeOutboundBuffer() were introduced in the early days of the new API when we did not have reference counting mechanism in the buffer. A user did not want Netty to free the handler buffers had to override these methods.

However, now that we have reference counting mechanism built into the buffer, a user who wants to retain the buffers beyond handler's life cycle can simply return the buffer whose reference count is greater than 1 in newInbound/OutboundBuffer().
[#1309] Make sure ReplayDecoder respect isSingleDecode()

* This could cause for example corrupt WebSocketFrame's if they was written from the server
  to the client directly after it send the handshake response.
Fix test failure in testsuite-osgi-deps

This commit fixes both failure of test itself and failure of compiling
and running test.

- When the test was run via 'mvn test', Maven gives karaf-maven-plugin a
  list of class directories instead of OSGi bundles, so that
  karaf-maven-plugin generates incorrect feature.xml.  I added a
  workaround for this specific case to DependencyIT
- When the packaging of project is 'feature', maven-compiler-plugin is
  not run at all.  Added a <plugin/> section so that it's always
  compiled.