minor optimization in Jsonrpc
minor optimization in Jsonrpc

Summary:
`Lwt_unix.of_unix_file_descr` does an `fstat` to determine the type of socket in order to set the blocking mode. we should only do that once instead of every time we use the fd.

Changelog: [internal]

Reviewed By: samwgoldman

Differential Revision: D36324095

fbshipit-source-id: 6a87fdfeb68e56ea9159952a04fa460e911a2cc4

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