perf: move to a single, locked writer database connection

[drahnr]

Currently we use a connection pool for all access, this includes the cleanup process (#1296) as well as apply_block. We should use a lock and retain the guard across the tasks for the cleanup to avoid any possible contentions.

[drahnr]

With the changes to #1725 we will have concurrent writes to two tables, so this needs more thought if we will have a per table / table set write connection that can exist at any point in time or if we drop the idea entirely

[Mirko-von-Leipzig]

I made a suggestion on how we can retain a single writer mode #1725 (comment):

I have an alternative which we can discuss and pursue in a follow-up PR.

We can add another Watch channel which is the inverse of the apply_block::chain_tip --> proof_scheduler one. The proof_scheduler never updates the DB itself, it just sets the latest proven block in the watch channel. On every apply_block, we update the proven block as well.

This adds a bit of latency to the marking, but considering the latency itself is 30s+, adding another 3s (worst case, avg 1.5s) doesn't seem too bad.

Whether this is worth it 🤷 I do like isolating database writes so we know for sure there are no problems.