store: implement schema migration

[philips]

We need to have a schema migration system in place. We should create the first migration for v0.6.0 that simply updates the version number or something.

Thoughts?

Based on this TODO: https://github.com/coreos/rocket/blob/master/cas/cas.go#L124

[shutej]

The liquibase project has diff tools and an updateSQL command. Our workflow has involved using external tools to prepare migrations and some go:generate magic to package them.

[sgotti]

Opened PR #738 (see also the considerations in the PR description). I tried to make it easy to write tests for multiple starting data and versions.

[jonboulle]

Initial version landed via #738, follow up in #748