SimpleMessenger: Pipe: do not block on takeover while holding global lock

We previously were able to cause deadlocks:
1) Existing pipe is fast_dispatching
2) Replacement incoming pipe is accepted
  *) blocks on stop_and_wait() of existing Pipe
3) External things are blocked on SimpleMessenger::lock() while
  blocking completion of the fast dispatch.

To resolve this, if we detect that an existing Pipe we want to take over is
in the process of fast dispatching, we unlock our locks and wait on it to
finish. Then we go back to the lookup step and retry.

The effect of this should be safe:
1) We are not making any changes to the existing Pipe in new ways
2) We have not registered the new Pipe anywhere
3) We have not sent back any replies based on Messenger state to
   the remote endpoint.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>