objecter: _get_session() returns -EAGAIN if lost race

Also, relevant callers restart in that case.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
objecter: don't blindly promote lock

In some cases the lock might already be promoted, check to see whether
it's already write-locked.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: _maybe_request_map() returns -EAGAIN if lost in race

Also modify callers to handle that if needed.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>