[torch/elastic] Revise the rendezvous handler registry logic.
Summary: Improve the implementation and the unit test coverage of `RendezvousHandlerRegistry`.
Test Plan: Run the existing and newly-introduced unit tests.
Reviewed By: tierex
Differential Revision: D27442325
fbshipit-source-id: 8519a2caacbe2e3ce5d9a02e87a910503dea27d7
