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>
msgr: clear con->pipe inside pipe_lock on mark_down

We need to do this under protection of the pipe_lock.

Signed-off-by: Sage Weil <sage@inktank.com>
msgr: reaper: make sure pipe has been cleared (under pipe_lock)

All paths to pipe shutdown should have cleared the con->pipe reference
already.  Assert as much.

Also, do it under pipe_lock!

Signed-off-by: Sage Weil <sage@inktank.com>