Objecter: check the 'initialized' atomic_t safely

shutdown() resets initialized to 0, but we can still receive messages
after this point, so fix message handlers to skip messages in this
case instead of asserting.

Also read initialized while holding Objecter::rwlock to avoid races
where e.g. handle_osd_map() checks initialized -> 1, continues,
shutdown() is called, sets initialized to 0, then handle_osd_map()
goes about its business and calls op_submit(), which would fail the
assert(initialized.read()) check. Similar races existed in other
message handlers which change Objecter state.

The Objecter is not destroyed until after its Messenger in
the MDS, OSD, and librados, so this should be safe.

Fixes: #9288
Backport: giant
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>