Improve IdentityStore Invalidate performance

[marcboudreau]

The Invalidate method of the IdentityStore struct was using a simplistic algorithm to synchronize the MemDB records (entities, groups, local entity aliases) with those from the storage bucket. This simplistic algorithm would result in a large number of MemDB operations within a single transaction whenever the storage bucket contained a large number or records. This large number of operations led to using a much slower comparer function within MemDB which caused the Invalidate function to take a long time to complete and could lead the node to fall so far behind in processing WALs sent over by the primary cluster that the replication state would transition to merkle-sync.

The simplistic approach basically consisted of deleting everything from MemDB that was associated with the invalidated storage bucket and re-inserting those resources using state contained in the storage bucket. Since invalidations usually occur to signal a single resource has changed, been added, or been deleted; when a large number of unchanged resources also exist in the storage bucket, a lot of unnecessary work was being done (deleting and re-adding).

These changes replace the simplistic approach for the handling of entities and local entity aliases since they are the more likely resource to exist in large numbers where this problem occurs.

The new approach consists of comparing the contents of the invalidated storage bucket with the set of resources from MemDB associated that storage bucket. Resources that match in both systems are left alone, and only differences are rectified in MemDB.