Do not send channel_id values over the wire

[remyers]

A Lightning channel_id is 32-bytes and takes up bytes 3-34 of most messages.

Once a channel has been established, these bytes can be inferred and do not need to be sent over the wire with each message; there is only ever one active channel per peer when using Mesh (and perhaps in normal Lightning).

ht/ @Snyke who searched through c-lightning and noticed 14 redundant instances of channel_id in peer messages.

In LNProxy we could include a switch on message type and strip out the channel_id before sending the message. On the receiver side the channel_id can be reinserted with a similar switch statement.