[Macro] - Persistence Mempool

[shenkeyao]

[Pating from Zulip discussion: https://espresso.zulipchat.com/#narrow/stream/419378-Block-Builder/topic/Persistence.20mempool/near/464430135.]

Instead of (or in addition to) keeping the list of transactions in memory, we should keep them in a database, so if the builder restarts for any reason we don't lose transactions. The property we want is that any transaction successfully submitted to the builder (submit endpoint returns 200) will eventually be included in a block.