librbd: make ImageCtx->object_map always present

librbd: make ImageCtx->object_map always present

This simplifies locking by obviating the NULL checks.  We no longer
need md_lock to protect these acceses. We can use object_map_lock
instead, to make sure no one reads an object map while its being
updated.

Keep track of whether the object map is enabled for a given snapshot
internally. In each public method, check this state, and automatically
set it correctly when refreshing the object map. During snapshot
removal, unconditionally try to remove the object map object, to
protect against bugs leaking objects, and to be consistent with image
removal.

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