Note transport: port database

Port the database changes to our new (empty) note transport layer crate.

This could be either the current sqlite, or the proposed postgres implementation - but not both. The latter does have scaling and durability benefits, though does incur latency costs.

The initial design refactor should reflect that we are now storing _committed_ notes only. This might have implications, in particular it may make sense to also store inclusion proofs though we can always add those later when/if the RPC endpoints dictate it.