Implement big transaction handling for chunk migrations, refs #100

This creates the local.migratelogref.sh collection, which is managed by a SpillableVector.
Also, chunk migration is aware of this collection and properly uses the referenced objects in _transferMods.

Soon, if SpillableVector proves correct and is the right abstraction, it could supersede TxnOplog for replication.
increase timeouts for moveChunk phase where we may be transferring many modifications, refs #100
add a test that stresses the migratelogref system, closes #100
re-enable zbigMapReduce because it was ok probably after #213