CA-405971: avoid calling DB functions when in emergency mode

[edwintorok]

This breaks emergency mode commands.

[edwintorok]

We might need to extend the condition here to avoid making these calls in general if we not on the coordinator, and instead use "baggage" headers to propagate the scheduling bucket, instead of querying it (which would introduce additional latency), but the change here should avoid the bug (and it'll also disable this code unless the feature flag is enabled)

[lindig]

You might want to add a comment that motivates the condition

[edwintorok]

discussed with @mg12 that it'd be good if we tweaked the buildsystem such that the Db module is not available in the emergency mode dispatch path at all (but that means we can't do the thread classification at all, at least not this early in the call tree). Will look at that separately once the blocker is fixed.

[edwintorok]

xe pool-emergency-transition-to-master finished successfully now.