[RPC] Refactor rref_context to not use utils::Future (#51697)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51697
Refactors the rest of rref_context, specifically pendingOwners map and `getOwnerRRef` to use JitFuture.
ghstack-source-id: 122037611
Test Plan: CI
Reviewed By: wanchaol
Differential Revision: D26243268
fbshipit-source-id: ab8874c8253274e8fe50dcd7291e0655a8f3f1df
