Add the ability to broadcast gossip msgs via the event pipeline

Add the ability to broadcast gossip msgs via the event pipeline

When we process gossip messages asynchronously we may find that we
want to forward a gossip message to a peer after we've returned
from the existing `handle_*` method. In order to do so, we need to
be able to send arbitrary loose gossip messages back to the
`PeerManager` via `MessageSendEvent`.

This commit modifies `MessageSendEvent` in order to support this.