Add more peers only to the mempool reactor

[evan-forbes]

Since CAT is both slow to distribute txs and very efficient, it would be highly beneficial to add more peers to this reactor. This would reduce the number of hops needed to distribute data along with increasing the scalability of the network. However, we do not want to blindly add many peers to other reactors, for example the consensus reactor, which has has issues with having many peers in the past.

This issue is blocked by specifying how we want to manage adding peers only with a subset of reactors. This could be quite simple at first, but we might need more advanced functionality such as having the option to increase the reactors which two peers are using to communicate, or we might want to have an entirely separate set of persistent peers that are only CAT.