msg/SimpleMessenger: drop msgr lock when joining a Pipe

8891 came up on normal runs.  I was able to finally track this down when
testing wip-msgr, though; I suspect something with the changes there make
the race a bit easier to hit.  In any case, the bug was introduced with
the fast dispatch and seem unrelated to wip-msgr.  And my wip-msgr test
which reproduces the hang after ~10 attempts now passes reliably.