Add mechanism to notify threads when om double buffer flushed

In order to support bootstrapping followers, the om snapshot subsystem needs to make consistent copies snapshot, including the deleted keys table which can be updated at any time.

This means that the background processes modifying the deletedKeys tables need to be halted, and then the bootstrapping process needs to wait for the double buffer to be flushed, (so that any remaining mods to the deletedKeys table is up to date.)

This PR adds the mechanism to allow a thread to wait until the double buffer has been flushed.