microoptimize Jsonrpc.has_message
microoptimize Jsonrpc.has_message

Summary:
assuming it's cheaper to test whether a Queue is empty than whether the socket is readable

Changelog: [internal]

Reviewed By: samwgoldman

Differential Revision: D36513804

fbshipit-source-id: aa0e85ab4c23909ad4e7cf012b4a94ef7066d579

mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow