rados: protect ctx.manager with a lock

Avoid race conditions by protecting the assignment of ctx.manager with
the global gevent lock.

http://tracker.ceph.com/issues/9027 Fixes: #9027

Signed-off-by: Loic Dachary <loic@dachary.org>
It is usually better to use locks as context managers, i.e. to do `with ctx.gevent_lock:`