Proposal: Subscription based mempool

[evan-forbes]

this is building on ideas from @mcrakhman and his investigation to the mempool slow downs

When users use the same account to sign over messages, this is incredibly slow due to the number of round trips along side the inability to distribute blobs out of order. This results in almost of the data from that signer taking the same path, meaning we are not able to take advantage of any congestion control.

getting congestion control would be possible if we had a way to prevent spam, such as guaranteed payment for txs, however that would likely require a consensus breaking change.

We could still reduce the number of round trips via subscriptions. Since nodes are downloading data within the same path anyway, we could assume that they want the blobs from the same signer for some period of time. if nodes observe SeenTxs from that signer from other nodes, they could opt to switch their subscriptions.