Add set if non-existant operation

A transactional operation to only set a value if does not exists would be useful. When this could be used, it would avoid a read/get in the transaction. This would be useful for a load transaction that checks if something does not exist. It would also be useful for implementing a queue.

I think this could be implemented in the 1st phase of two phase commit when locking row/columns. The conditional mutation that acquires the lock could do the empty check as part of its other checks.