Summary: We're using StringKeyed* from common/datastruct to avoid unnecessary string creation whenever we're looking up string keys. C++14 does offer a solution, see e.g. http://stackoverflow.com/questions/10536788/avoiding-key-construction-for-stdmapfind. That is not supported by current compilers.

Test Plan: unittests

Reviewed By: pavlo@fb.com

Subscribers: trunkagent, net-systems@, folly-diffs@, yfeldblum

FB internal diff: D1825700

Signature: t1:1825700:1423086724:530550c3c80e33c80900f31c0ade05c66b22cbe8