DESC
Try to rollback when a transaction fails to commit
EDESC

When a transaction fails to commit, try to rollback it before returning. The
huge comment in the code says it all, read it.


While at it, fix a very small but theoretically possible race: we need to
unlink the transaction before freeing it. Otherwise, we could:

task 1			task 2

free_tid(5)
			get_tid() (returns 5)
			write the transaction file
unlink(5)

And we have just unlinked a valid transaction!