Fixes #237 (Empty ByteBuf leak)

The code was not sending an `onNext` call for empty buffers.
This resulted in leaking memory for empty payload responses as the release of `ByteBuf` was a part of the `onNext` call.

 This change sends the content `onNext` unconditionally and lets the user decide what to do with the buffer.
Merge pull request #242 from NiteshKant/0.x

Fixes #237 (Empty ByteBuf leak)
Fixes #243 (HttpServerListener requestReadTimes metric value is incorrect)

Now recording the request processing start time
Fixes #245 (Race condition between write and flush in `HttpClient.submit()`)

Flushing now only when the LastHttpContent is written on the channel.
Fixes #248 #249 #250 #251

#248 (Connection close cancels pending writes): Now only cancelling when a pooled connection is returned to the pool.
#249 (`BytesInspector` does not record events for `ByteBufHolder` and `FileRegion`): Recording events for ByteBufHolder and FileRegion
#250 (Connection closing and content stream behavior): Send error on content stream if the request/response is not completed on server/client.
#251 (Metric events for Request content source error and request write failure): Added metrics
Fixes #252 (Upgrade to RxJava 1.0.0-RC7)
Fixing broken rx-netty-context tests.
Fixes #229 (Remove deprecated method `DefaultChannelWriter.getChannelHandlerContext()`)
Updating copyright
Merge branch '0.x' of https://github.com/ReactiveX/RxNetty into 0.x
Merge pull request #253 from NiteshKant/0.x

Fixes issues for milestone 0.3.15
Merge branch '0.x' of https://github.com/ReactiveX/RxNetty into 0.x
Updated CHANGES.md for 0.3.15
Merge pull request #255 from NiteshKant/0.x

Updated CHANGES.md for 0.3.15
Upgraded gradle wrapper.

Fixed dependency of rxjava-math to suit netflix internal build.
Merge pull request #256 from NiteshKant/0.x

Upgraded gradle wrapper.
Manually updating revision to 0.3.16
Fixes #260 (Convert `RxEventLoopProvider` and `MetricEventsListenerFactory` to abstract classes)
Fixes #261 (ServerSentEventEncoder is sub-optimal)

Removed usage of StringBuilder, instead uses netty's `ByteBuf`
Fixes #258 (Add port to the host header)