Implement the sending of `start_batch` and `protocol_batch_element` from `channeld` to `connectd`.
Each real peer wire message is prefixed with `protocol_batch_element` so connectd can know the size of the message that were batched together.
`connectd` intercepts `protocol_batch_element` messages and eats them (doesn’t forward them to peer) to get individual messages out of the batch.
It needs this to be able to encrypt them individiaully. Afterwards it recombines the now encrypted messages into a single message to send over the wire to the peer.
`channeld` remains responsible for making `start_batch` the first message of the message bundle.
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