Flush for async cross-site (IRAC)

Add a new method to wait for the asynchronous cross-site replication to complete for a key (or set of keys).

This method will have 2 operation modes, wait until the key is not present in the queue or until the current value at the time of the invocation is flushed. The difference is that a key may be changed while waiting for it to be flushed and the second method would not wait for the later update.

A timeout would be required as well.