[MINOR] add the ability to force kernel socket buffer size.

[MINOR] add the ability to force kernel socket buffer size.

Sometimes we need to be able to change the default kernel socket
buffer size (recv and send). Four new global settings have been
added for this :
   - tune.rcvbuf.client
   - tune.rcvbuf.server
   - tune.sndbuf.client
   - tune.sndbuf.server

Those can be used to reduce kernel memory footprint with large numbers
of concurrent connections, and to reduce risks of write timeouts with
very slow clients due to excessive kernel buffering.