Add blocks table to database

This branch adds a blocks table to the database and migrates message bodies from the messages table to the blocks table. This is the first step in adding multi-block message support to the DB.

The migration and subsequent compaction takes a significant amount of time - about a minute on a slow phone with ~750 messages (the default when creating test data).

The branch also modifies the existing migrations to close their statements. As far as I know the statements would have been closed automatically when the respective connection was closed, so I don't think there was a significant resource leak.

Closes #1595