[rref] Minor tweaks in rref_context (#36419)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36419
Since we call waitForThreadLocalPendingRRefs per-RPC, construct it
already-satisfied in the common empty case, to avoid extra mutex/cv work.
Also, naming consistency for recording_.
ghstack-source-id: 101975739
Test Plan: ctr_mobile_feed, buck test mode/dev-nosan caffe2/test/...
Differential Revision: D20977879
fbshipit-source-id: e321a33127e4b5797e44e039839c579057e778e5
