added SpillableVectorIterator for querying a SpillableVector #782
added RATELIMITED for printing messages only once in a while #782
removed OldDataCleanup and some unused vars in MigrateFromStatus #782
reworked migrations to pull data using normal cursors #782

Added _migrateStartCloneTransaction to kick off the process, and used
normal cursors for the rest.  Also removed the fairly useless
flushPendingWrites function.
made startCommit fail faster if it needs to #782
made *_during_migrate.js more robust