Allow JsonClient tests to run in parallel
Allow JsonClient tests to run in parallel

Summary:
Remove hardcoded port from test and start using ListenSocket to allow multiple
test runs in parellel.

Reviewed By: spalamarchuk

Differential Revision: D5142216

fbshipit-source-id: c0e57d25b9e89c250e1d3511bbb05fe5e3c8ea5f

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter