Call `ChannelMessageHandler::message_received` without peer lock

Call `ChannelMessageHandler::message_received` without peer lock

While `message_received` purports to be called on every message,
prior to the message, doing so on `Init` messages means we have to
call `message_received` while holding the per-peer mutex, which
can cause some lock contention.

Instead, here, we call `message_received` after processing `Init`
messages (which is probably more useful anyway - the peer isn't
really "connected" until we've processed the `Init` messages),
allowing us to call it unlocked.