Avoid re-allocating to encrypt gossip messages when forwarding

Avoid re-allocating to encrypt gossip messages when forwarding

When we forward gossip messages, we store them in a separate buffer
before we encrypt them (and commit to the order in which they'll
appear on the wire). Rather than storing that buffer encoded with
no headroom, requiring re-allocating to add the message length and
two MAC blocks, we here add the headroom prior to pushing it into
the gossip buffer, avoiding an allocation.