feat: use recaller to persist the time of last scrape

[nadiamoe]

Replaces #1607, which I accidentally closed by pushing the wrong ref 😓

Builds on top of #1606

Closes #1605

[nadiamoe]

Answering to @ka3de's comment:

There is a parallel open PR that reworks the tenant's cache in order to use an external memcached (#1601). I think we should settle with either Redis or Memcached for both purposes.

I do not have a strong opinion on redis vs. memcached at this point. I do think though, that:

• It should be fine for us to play with both, I don't see a compelling reason to lock ourselves to one if we want to try out the other
• I will default to redis on the basis of us having the knowledge to operate it (as we do with the MQ subsystem), and it being HA and production tested already.

[nadiamoe]

From Dougal: Let's make sure we persist the time when the scrape started, but when the check finishes.