Encrypt+MAC most P2P messages in-place

Encrypt+MAC most P2P messages in-place

For non-gossip-broadcast messages, our current flow is to first
serialize the message into a `Vec`, and then allocate a new `Vec`
into which we write the encrypted+MAC'd message and header.

This is somewhat wasteful, and its rather simple to instead
allocate only one buffer and encrypt the message in-place.