Add support for running database replicas

[aspark21]

moodle-ci-runner does this already but I think it would be useful for dev environments to have this to more closely replicate prod.
noting this as we just had a piece code make it's way to prod that wasn't using transactions and was affected by the replication delay of reading from a replica just before deciding to write.

Seems worthwhile us providing a merge request for this & might look at #170 properly while we are at it in a couple of weeks