Get a read/write database handle to the blockstack db.
        At most one such handle can exist within the program.

        When the caller is done with the handle, it should call release_readwrite_instance()

        Returns the handle on success
        Returns None if we can't set up the db.
        Aborts if there is another read/write handle out there somewhere.

def borrow_readwrite_instance(cls, working_dir, block_number, expected_snapshots={}):
        """
        Get a read/write database handle to the blockstack db.
        At most one such handle can exist within the program.

        When the caller is done with the handle, it should call release_readwrite_instance()

        Returns the handle on success
        Returns None if we can't set up the db.
        Aborts if there is another read/write handle out there somewhere.
        """

        global blockstack_db, blockstack_db_lastblock, blockstack_db_lock

        import virtualchain_hooks
        db_path = virtualchain.get_db_filename(virtualchain_hooks, working_dir)
        
        blockstack_db_lock.acquire()

        try:
            assert blockstack_db is None, "Borrowing violation"
        except Exception, e:
            log.exception(e)
            log.error("FATAL: Borrowing violation")
            os.abort()

        db = BlockstackDB(db_path, DISPOSITION_RW, working_dir, get_genesis_block(), expected_snapshots=expected_snapshots)
        rc = db.db_setup()
        if not rc:
            db.close()
            blockstack_db_lock.release()
            log.error("Failed to set up virtualchain state engine")
            return None

        blockstack_db = db
        blockstack_db_lastblock = block_number
        blockstack_db_lock.release()

        return blockstack_db