Add Org Identity Source Processing Callback

Some OIS Plugins (FileSource, SqlSource as of v.4.3.0) maintain a cached set of processed records to allow a diff to be performed to efficiently determine the set of records that need to be processed. However, these plugins must update that cache as part of the changelist calculation since there is no other place to do so. (Both retrieve() and resultToOrgIdentity() are frequently called for read-only purposes, so it doesn't make sense to update the cache on those calls.)

A new callback should be added, either after the processing of each record (maybe less efficient) or after processing has completed (on partial update if the cache is not updated again already processed records might be reprocessed, but that shouldn't break anything).