Fixes for 0.3.16

Fixed the following issues:

#260 Convert `RxEventLoopProvider` and `MetricEventsListenerFactory` to abstract classes. This aids in adding new methods without breaking existing implementations.
#261 `ServerSentEventEncoder` is sub-optimal in using StringBuilder. Removed `StringBuilder` and using `ByteBuf`
#258 Adding port to HTTP host header.
#259 Support native netty protocol as a runtime choice.