Introduce incremental ticking

In input-output-hk/ouroboros-network#2590 we suggested the concept of incremental ticking. We didn't end up needing it for input-output-hk/ouroboros-network#2590, but it might still be useful in other contexts than block forging. I.e., the Mempool and chain selection.

If the mempool ticks the ledger state to the slot of the virtual block it is producing, that expensive computation might be repeated when we do chain selection or block production. So we'd have to shared the cached result between Mempool and ChainDB.

A more useful example is chain selection: if we toggle between two forks multiple times that happen to cross the expensive computation boundary, we'd compute it multiple times.