Fixes issues for milestone 0.3.15

#243 HttpServerListener requestReadTimes metric value is incorrect
#245 Race condition between write and flush in `HttpClient.submit()`
#248 Connection close cancels pending writes
#249 `BytesInspector` does not record events for `ByteBufHolder` and `FileRegion`
#250 Connection closing and content stream behavior
#251 Metric events for Request content source error and request write failure
#252 Upgrade RxJava to 1.0.0-RC7
#229 Remove deprecated method `DefaultChannelWriter.getChannelHandlerContext()`