Implement single trigger list for transaction state changes

There are currently two states of a transaction: commit and rollback, both have triggers. With synchronous replication there is a new event - successfully WAL write, but not commit yet. This is needed in the applier so as to send an ACK to the master when this happens.
Current solution uses a hack with waking up the applier's fiber using fiber_wakeup(). However this is not a perfect solution - the writer fiber may not only wait for being woken up after WAL write, but also may be busy with other work, such as still sending a previous ACK. Its preliminary wakeup in this case leads to unnecessary wakeup and going back to sleep.
This can be solved by merging on_commit and on_rollback transaction triggers into one trigger list. The trigger list would contain all the triggers together, with a mask of events they want to listen to: commit, rollback, WAL write. When the triggers are run, the event is checked against the masks.
This not only allows to remove the hack with applier, but also makes struct txn 16 bytes smaller.

Start by locating struct txn and the on_commit, on_rollback, and fiber_wakeup paths in the transaction and replication code. Trace how the applier and writer fiber handle WAL writes, commits, rollbacks, and ACKs. Done means one masked trigger list handles all three events, the applier wakeup hack is removed, and struct txn is smaller without unnecessary wakeups.