Add indices on Actions table.
Improve Actions indexes.
Merge branch 'master' of github.com:jsha/blocktogether into actions-index
Merge pull request #101 from jsha/actions-index

Add indices on Actions table.
Trim old blocks

Call destroyOldBlocks in finalizeBlockBatch to remove all but two latest
BlockBatches for user when a new BlockBatch is retrieved successfully.