Codify database invariants

[michaelsproul]

Description

There are several invariants maintained by the database which are not currently documented anywhere other than in comments and code. To improve this I think we should add a set of invariants that are:

• Executable
• Documented

These can then be used in a tool like lighthouse db check-invariants, in tests, and in periodic background tasks on some of our infrastructure.

I think it might be good to describe each invariant in a struct like:

Invariant {
   name: "freezer_block_roots",
   description: "Every slot from oldest_block_slot to the split slot should have a block root",
   speed: InvariantSpeed::Fast,
   check: |db| db.iter_column(DBColumn::BeaconBlockRoots).for_each(...),
}

Version

Lighthouse v4.5.0

Additional Info

Would hopefully help catch issues like #4697 earlier in the development cycle.

[michaelsproul]

Assigning this to myself because I've probably got the most DB invariant context, but I'm not making a firm commitment to work on this immediately.