[FTS] moss merger should do full merges while waiting for persistence

Moss store background compactor will only compact segments when the ratio of the sizes of the consecutive segments exceeds MinMergePercentage which is set to 0.8 by default.

This results in multiple segments being sent over to the persister.

An optimization is that while waiting for the background persister to finish, the collection merger can just do full merges so that the reads & future compactions are optimized.