remotefilelog: implement threaded _getfiles

Summary:
A better way to avoid deadlocks and not sacrifice performance on _getfiles
call.

Test Plan:

• build, pull and update on Windows
• build, pull and update on Linux
• do not observe it hanging

Reviewers: durham, #fbhgext

Differential Revision: https://phab.mercurial-scm.org/D1467