http: unref socket timer on parser execute

When underlying `net.Socket` instance is consumed in http server - no
`data` events are emitted, and thus `socket.setTimeout` fires the
callback even if the data is constantly flowing into the socket.

Fix this by calling `socket._unrefTimer()` on every `onParserExecute`
call.

Fix: #5899
PR-URL: https://github.com/nodejs/node/pull/6286


Reviewed-By: James M Snell <jasnell@gmail.com>