NEW Allow database read-only replicas

[emteknetnz]

Issue #11352

[emteknetnz]

I'm enforcing must_use_primary on anything "Security" related - i.e. any url that matches /Security, any DataObject under the Security namespace, to prevent any weird issues that could happen when a replica is out of sync with the primary

For instance a password is changed, the user logs out, but cannot log back in because the replica isn't synced up yet.

Having replicas be slightly out of sync is one of the trade-offs for using replicas, though for anything security related it introduces risk so it's disallowed

[GuySartorelli]

Haven't tested locally yet but here's some requested changes based on reviewing the code

[GuySartorelli]

• NEW Allow database read-only replicas #11379 (comment) still hasn't been addressed
• New comment in NEW Allow database read-only replicas #11379 (comment) (sorry, forgot to start review)

[GuySartorelli]

Noticed a couple of changes which make it seem like this PR probably wasn't updated fully when you retargetted 6? Please double check before I continue my review.

[GuySartorelli]

There's also some failing CI - does some of that rely on other PRs?

[emteknetnz]

I think the unit test failures are related to the branch having /5/ in it when it's a CMS 6 PR (originally it pointed to CMS 6) - our CI looks at the branch pattern to get the version of installer

Issue links to a recipe sink build so we can rely on that

[GuySartorelli]

LGTM