transaction: support multiple, separate transactions Solves that committed (closed) transactions may linger and be returned on subsequent transaction calls, even though a new transaction should be made, rather than a new nested transaction. This also fixes a race condition with the use of weakref.

transaction: only delete journal on successful abort/commit This solves that the journal file was always deleted when the transaction was deleted, no matter whether the abort (rollback) succeeded or not. Thus, never supporting a hg recover. The journal file is now only deleted on close (commit) or a successful abort.