Resource leak and keep alive fixes for netty

The first commit fixes a resource leak I discovered using the resource handler for files when the file's last modification time matches the If-Modified-Since header.

For the sake of reuse I thought it would be a good idea to expose a method returning a ChannelFutureListener that releases the netty request once it has been dealt with.

The second commit keeps the connection open for 304 HTTP status responses if possible. I cannot see that closing down the connection is required in any way per the RFC. A quick check on some famous web servers revealed that they keep the connection open, too.