Provide a hook for obsutil.getobsolete to be used with either a
transaction or the changes item of the transaction, since hooks only
have access to the latter. Use that to find the correct list of
revisions with obsmarkers, even new ones, and then filter out revisions
with known successors.
Move the processing from pretxnclose to txnclose as the transaction
access itself is no longer necessary. This is more in line with notify
and ensures that sanity checks can abort the transaction first.