Problem/Motivation
getMessageIterator() formerly retrieved the source IDs for each message - however, as a result of #2613878: Use hash for Migration source keys, rather than verbatim values the message table no longer contains those IDs in favor of a hash.
Proposed resolution
Pull the source IDs from the map table and return those instead of the hash.
Remaining tasks
Write tests.
User interface changes
N/A
API changes
None
Data model changes
None
Original issue summary (from migrate_tools queue)
drush mmsg displays the columns of the messages table - however, we're now using a hash of the source IDs as a key, which is not very useful. We should pick up the real source IDs from the map table and display those instead.