msgr: clear_pipe inside pipe_lock on mark_down_all

msgr: clear_pipe inside pipe_lock on mark_down_all

Observed a segfault in rebind -> mark_down_all -> clear_pipe -> put that
may have been due to a racing thread clearing the connection_state pointer.
Do the clear_pipe() call under the protection of pipe_lock, as we do in
all other contexts.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 5fc1dabfb3b2cbffdee3214d24d7769d6e440e45)

Conflicts:

	src/msg/SimpleMessenger.cc