Connection Pool refactoring.

* Modified Connection pool interface to also send PipelineConfigurator to acquire(). This is required as the pipelineConfigurator is modified by the client to accommodate all RxNetty specific handlers. Storing the pipeline configurator provided by the user does not work for the pool.
* The shutdown of eventloop during client shutdown will shutdown the shared eventloop (by default). Added ref counting on the shared eventloop group, so it only shutsdown if there are no more references.
* Removed shutdown() call from waitTillShutdown() in RxServer as it is used as a waiting mechanism for an external shutdown for a server.
* Enabled ClientRequestResponseConverter to be reused by resetting the contentSubject.
* Added more tests and fixed the existing tests.