fix: reliable key exchange

[hackergrrl]

This fixes a race condition where the key and other multifeed data could end up
being read together (one big buffer vs multiple smaller buffer).

[hackergrrl]

This covers the race condition case where

1. I send you my key, and you send me yours
2. I receive yours, and begin multifeed replication, which means sending the multifeed header
3. this maybe happened quite fast on my end, so my kernel decides to collate my key and the multifeed header together, and send both as one packet over the network
4. the data event fires, and the buffer has the remote key and some/all of the multifeed header, resulting in either a bad handshake or the multifeed header being discarded