Use SrcKey in RegionContext instead of its components
Summary:
RegionContext struct was carrying all[*] the fields that SrcKey already contains
and its users were often reassembling SrcKey from these fields.
Let's just use SrcKey directly in RegionContext.
Makes it possible to extend the functionality of SrcKey and let the information
propagate.
[*] except of the prologue flag, but fortunately it was not used
Reviewed By: ricklavoie
Differential Revision:
D18131177
fbshipit-source-id:
345552b6de4390bd4ddaac452c02e93cf133d983