Track DB changes in CI

Currently, it is easy to mistakenly make breaking changes to the DB schema. We should make this a little harder. One way is to check this via CI.

One possible strategy for doing this:

* Retain old copies of SQL schema somewhere in CI
* Run `go-livepeer` on each old schema
* Ensure `go-livepeer` starts cleanly
* Hopefully the above process will catch any breakage. From there, we can check in a new copy of the schema for any breaking changes that requires a bump to the `LivepeerDBVersion`.

https://github.com/livepeer/go-livepeer/blob/33fa9b29960d34193e39e18aba0052e8ffded8fb/common/db.go#L50

Start with common/db.go around line 50 and inspect how LivepeerDBVersion and the SQL schema are handled. Then trace the existing CI entry points for running go-livepeer. Done means CI retains prior schemas, starts go-livepeer against each one, detects startup failures, and supports checking in a new schema when a version bump is required.

Written by the indexing model from the issue text.