Implement transactions

Implement transactions

New parameters:
    --start-transaction: Creates a copy of the state of the database, for
	use with --transactional. This overrides any previous uncommited
	transaction. Should only be used after --lock-for
    --commit-transaction: Atomically moves the copy back to the main, thus
    	commiting the changes
    --transactional: Flag to indicate that we want to work on the copy