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.
I would simplify this by leaving out the BtUser.find. Just pass the source_uid to destroyOldBlocks, and it can do a

```
BlockBatch.findAll({
  where: { source_uid: source_uid }
}
```