cls_journal: new tag management methods and handling

cls_journal: new tag management methods and handling

In the case of librbd, a new tag will be allocated when the
exclusive lock is acquired.  All tags for the same dataset
(e.g. librbd image) will belong to the same class.  Tags are
automatically pruned on tag create / client unregister
if no other clients' commit position would require the tags.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>