We try to set `max_statement_time` on new database connections so that long queries can be interrupted server side. However, the existing code is broken because of connection pooling. The `SET SESSION max_statement_time=...` gets executed on on connection, but subsequent queries won't necessarily be executed on the same connection.
This changes fixed the problem by introducing an alternate DB driver that wraps the MySQL driver to prefix every query with `SET STATEMENT max_statement_time=...`
This also changes `vars.go` to use the DSN form of a database name instead of the URL form. This allows using it directly in `prefixed_db`'s tests, and this is the direction we're moving all of our database URLs. We previously used a homebrewed URL syntax because it allowed us to extract certain fields and set config options, but now `mysql.Config` serves that need.
Fixes#2251
Jacob Hoffman-Andrews