Controllable block producer #792

Block and batch production is currently automatic, every M seconds a batch build is triggered, and every N a block.

This makes it difficult to get the chain into a specific state since batches and blocks are somewhat random. It would be useful for integration tests, demonstrations and test tooling for builders if this was more controllable.

I propose an additional manual block-producer mode. When launched in this mode, the block-producer does not spin up batch and block producer tasks, but instead launches a server to accept commands from.

The first concrete step of this issue should be a API design, but I imagine we could start with a simple /build_batch and /build_block. Maybe we want to allow specifying number of transactions/batches to select, or even which ones to include.

It would also be ideal if the user of this API did not need to know about its specifics, and could instead use CLI to control it. This should be described in the initial design proposal.