Better control over database migrations

[Thegaram]

We use migrations to allow for the evolution of the rollup-node database over time. The migrations are run automatically on node startup.

While running the migrations on startup is the best option for 3rd-party node operators, during testing and development we often need more control over these. I suggest two changes:

1. Add a flag --disable-migrations that prevents the node from running the migrations automatically.
2. Expose cli commands that can be used to run and rollback migrations manually. For example, Lighthouse has this command: lighthouse db migrate --to 16.

Implementing this can be combined with #297 which proposes another related improvement.