Implement mcrouter route for routing to master region
Implement mcrouter route for routing to master region

Summary:
TAO requests contain a field that specifies the master region name, called
"masterRegion". The MasterRegionRoute uses the master region name to look up the
route handle to route the request to.

Reviewed By: davidgoode

Differential Revision: D7312972

fbshipit-source-id: 7cca4b0c70dac0a8eadb663fa50869b8f7d527fd

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