added SpillableVectorIterator for querying a SpillableVector #746
removed OldDataCleanup and some unused vars in MigrateFromStatus #214 #746 #823

fixes #214
fixes #823
reworked migrations to pull data using normal cursors #746 #824

Added _migrateStartCloneTransaction to kick off the process, and used
normal cursors for the rest.  Also removed the fairly useless
flushPendingWrites function.

fixes #824
made startCommit fail faster if it needs to #746
made *_during_migrate.js more robust #746