Trim Old Blocks

Addresses #59 

When a new BlockBatch is fetched in `updateBlocks`, trim BlockBatches in DB to only include 2 most recent for this BtUser.
Deleting BlockBatch items will clean up Blocks table automatically via cascade.
destroyOldBlocks should only get called when we are finished with a batch and set the 'complete' flag. Otherwise we can wind up with the most recently collected batches being incomplete ones.