Calls doAfterCompletion in TxManager#commit/rollback

since vital functions happen there, for example lock releasing. Previously
there were a scenario where a transaction started and ran into a "tm not OK"
exception on commit, where the after completions wouldn't be called.
This might happen if the tm not OK came from a kernel panic and a handler
brought the db back on its feet again.
Merge pull request #2234 from tinwelint/1.9-after-completion-fin

Calls doAfterCompletion in TxManager#commit/rollback