Reorganize logic with actors

[jeromegn]

Turns out we're spinning up a lot of unsupervised tokio tasks. These tasks need to run and keep running, but they can crash in very rare circumstances.

It would be more ergonomic to use an actor system for these long running background tasks.

Examples are:

• Bookkeeping compaction
• "Cleared" versions DB writes
• Synchronization loop
• Partially buffered changes procesing
• ... many more!