Close multiplexer in a strand to prevent races
Removes this_thread::yield() from ConnectionManager::Close

I was not seing any test failures without it so
there is a chance the problem this was fixing has been
solved. If not, I would like to see the original problem,
perhaps to fix it differently.
Merge pull request #49 from inetic/MAID-471-thread-sanitizer-warnings

Maid 471 thread sanitizer warnings
Merge remote-tracking branch 'upstream/next' into MAID-537