msgr: drop SimpleMessenger::lock while doing local_dispatch

There are a bunch of ways this could deadlock now that we have
fast_dispatch to deal with. Hopefully we can drop even invoking
this lock here soon.

Signed-off-by: Greg Farnum <greg@inktank.com>
SQUASH "OSD: do not take pre_publish_lock in send_message_osd_cluster()"

Do the same thing in get_con_osd_cluster() and get_con_osd_hb().

Signed-off-by: Greg Farnum <greg@inktank.com>