Revert "librbd: use task finisher per CephContext"

Revert "librbd: use task finisher per CephContext"

Since notify handling was made async from the librados threads in
d898995b0e3ea301b1325f68a0532d57afa3c816 tests can crash during
image close when exclusive locking is enabled.

This occurs because flushing the watches no longer guarantees that all
notifies have been completely handled, and since these are run from
the TaskFinisher attached to the CephContext, notifies added to the
TaskFinisher run after the ImageCtx they refer to has been
destroyed. The notify for exclusive lock release runs into this in
this case.

Looking into this also made me notice that sharing a single
TaskFinisher is not safe currently since all events are cancelled by
ImageWatcher::unregister_watch(), not just those scheduled by that
image.

Example crash backtrace from test_rbd.py:

    at librados/IoCtxImpl.cc:1332

This reverts commit 96563c15159d1ba0e0978e76b8df6a8ab311e5d2.

Fixes: #14780
Signed-off-by: Josh Durgin <jdurgin@redhat.com>