Trim old blocks

Call destroyOldBlocks in finalizeBlockBatch to remove all but two latest
BlockBatches for user when a new BlockBatch is retrieved successfully.
Call destroyOldBlocks with uid instead of BtUser object
Merge pull request #100 from BooDoo/issue/59/trim-old-blocks

Trim Old Blocks
Session table proof-of-concept
(needs at least a migration script for adoption)