SnapDiff job and compaction DAG/SST file pruning synchronization

Currently, it is possible that when snapDiff job is running we may loose some of compaction DAG if snapshots timing is overlapping with time by which snapshot becomes stale in compaction DAG.

For example, if one or both of the snapshots of snapDiff job are 30 days old and compaction DAG pruning service is removing 30 days older snapshots, we could be in that situation.

This task is to introduce the read/write synchronization in compaction DAG.