net: require a verack before responding to anything else
net: require a verack before responding to anything else

7a8c251901 made this logic hard to follow. After that change, messages would
not be sent to a peer via SendMessages() before the handshake was complete, but
messages could still be sent as a response to an incoming message.

For example, if a peer had not yet sent a verack, we wouldn't notify it about
new blocks, but we would respond to a PING with a PONG.

This change makes the behavior straightforward: until we've received a verack,
never send any message other than version/verack/reject.

The behavior until a VERACK is received has always been undefined, this change
just tightens our policy.

This also makes testing much easier, because we can now connect but not send
version/verack, and anything sent to us is an error.

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