pcs: Use if-unmodified-since to avoid unnecessary rendering

[johngian]

Replicate what Parsoid on RESTBase is doing to avoid unnecessary rendering from template changes.

• From what it looks from prod metrics we do get changeprop requests that the job date is for content that has already been superseded (old events)
• Currently parsoid is doing all the heavylifting that checks the if-unmodified-since and if the events are stale doesn't propagate more resource change events
• By disabling pregeneration on parsoid and replacing the pregeneration trigger for PCS/summary with MW resource changes we end up with the same problem (and increased load).
• This patch is replicating the logic from parsoid to PCS/Summary endpoints

[johngian]

I think now with the fix for summary endpoint this is ready for review @brightbyte