Consolidate cache TTL handling within CoreMagicVariables.
Make the TTL account for how many seconds away the value is from changing.
For example, CURRENTHOUR should change soon after the next hour is reached.
There is a minimum adjustment TTL to avoid parser-after-save delays.
This allows for longer caching in most cases, as well as more up-to-date
rendering when the hour/day/week/year is about to change. Previously, there
were blind TTLs, which are either way too pessimistic or way too generous.
This commit does not change the CURRENTTIME, CURRENTTIMESTAMP, LOCALTIME,
and LOCALTIMESTAMP words, since there is no reasonable way to cache output
while keeping them up-to-date.
Bug: T320668
Change-Id: I9acb42b0d9ff67798a1624cbf9c7cac99c8fbe2f
