[1.x] perf: store scheduler timestamp in cache

[imorland]

Fixes #0000

Changes proposed in this pull request:
Currently when the scheduler is run, it stores the last run timestamp in the DB (settings). This PR changes that to store in the cache, which is faster, and decreases the load on the DB

Reviewers should focus on:

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)