user: Add mysql user db implementation.

[lukebp]

The backend database choice in limited by the tlog mysql dependency. A mysql user database implementation should be added so that we are not running multiple databases on the backend.

This issue also needs to include a migration path from the existing cockroachdb to mysql.

[amass01]

Will take this one

[xaur]

The backend database choice in limited by the tlog mysql dependency.

This answered my old queued question "why MySQL?". For anyone interested, here is the discussion of PostgreSQL support in the Trillian repo: google/trillian#1298