Improve background cleanup of persistence actors to be more resource efficient

[thjaeckle]

Under certain scenarios it is not ideal that Ditto loads Things/Policies Actors into memory regularly just for the "Background cleanup" (of event journal and snapshots).
E.g. when using Ditto for "Things" which are:

• rarely updated (e.g. only once a month)
• rarely accessed (e.g. only by few users via a frontend)
• but are e.g. big in payload size
• or are very many (millions)

Some ideas:

• Don't load PersistenceActor in memory if no cleanup is required
  • E.g. by filtering those (things/policies) which do not have recent changes out directly in the "cleanup stream"
  • Ideally not even sending any message to the PA (which would recover it from the DB)
• If a PA was loaded into memory just for cleanup:
  • Shut it down a lot earlier than "normally" (2 days is the default for things in Helm config) to free up memory
  • Make it configurable (if enabled and how long those shall be kept "live")