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