background span policy enforcer

[RaduBerinde]

We currently allow overriding various settings for specific key spans via a span policy (most importantly, tiering will be controlled via the span policy).

We need a background process that periodically rechecks the span policies and triggers any compactions necessary to enforce the policy. It should also enforce compression settings and file sizes.

I see it as something that goes across the keyspace, throttling itself with a target of going once over everything every X hours. Perhaps the idea of the "cursor" used by download compactions can be reused here.

Jira issue: PEBBLE-1302