netty: simplify releasing of ref-counted messages

Add a `releaser` ChannelFutureListener to the netty specific
ReceivedMessage binding and put it to use.

This fixes a resource leak of a ByteBuf where a NettyHttpRequest was not
freed in case a 304 HTTP response was sent to the client for a given
file resource, cf. netty/netty#2502.
do not close the connection for HTTP 304 responses

Adhere to the client request or use the default keep-alive setting of
the HTTP protocol version.
Merge pull request #222 from avdv/resource-leak-and-keep-alive

Resource leak and keep alive fixes for netty