feat: allow multiple blocks per batch

[itegulov]

Current implementation seals a batch after every single miniblock (and hence after every single tx). Motivation to resolve this is very similar to what is outlined in "Allow multiple txs per block" but there is an extra benefit in that we will stop producing empty blocks after every single real block. Which results in latest block never having any txs in the current implementation.

I propose we allow two modes of operation:

• Manual batch sealing via debug_sealBatch (just an example, namespace + name to be decided)
• Automatic batch sealing every so often (configurable on startup)

NOTE: A major problem with this is that we can't mix impersonated and non-impersonated transactions inside a single batch due to limitation of our current approach (batch VM is initialized with either normal or impersonated transactions at the start of a batch).

[itegulov]

Marking this as blocked as I feel like the design here is heavily dependent on #407