storage/mysql: support pre-5.7.20 instances with tx_isolation only

[bonifaido]

MySQL 5.7.20 introduced the transaction_isolation system variable, so pre 5.7.20 versions won't work with Dex MySQL storage (like AWS Aurora currently). This PR adds some basic detection logic for those use cases. See #1485 (comment) for more details. Once we can't get rid of the SERIALIZABLE requirement, this logic can be removed.

[srenatus]

LGTM. Haven't tried this, but the code change looks alright.

[JoelSpeed]

My only concern is whether this is likely to break in the future if that error in particular is reused? Is there any other way to tell about the version and the compatibility we need to configure?

[bonifaido]

We could do a db.Query("SELECT VERSION()") and compare the semantic versions with github.com/coreos/go-semver (that lib is an indirect dependency in the project already).

[bonifaido]

Merging this, for now, later on we can refine this check if any problems occur.