Migration meta issue

[ValuedMammal]

We should document a process for handling database migrations.

Requirements

• Support both up and down migrations. This may require maintaining the old queries.
• Try to batch schema changes into a single migration
• Migrations should happen in a major version release

Known issues

• block table: Only the height should be unique. Currently the block table has a primary (height, hash) constraint. This could present a problem if two rows contend for the same height, because the local chain changeset only supports a single blockhash per height. (note: it might not be a huge concern, since the changeset also supports deleting rows of reorganized blocks).
• How will the schema look in a multi-keychain world?