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