Need to relax temporary_channel_id check

[yuntai]

Per
https://github.com/lightningnetwork/lightning-rfc/blame/4b62d26af93a07166d6a9de2cb5eff80849d9c19/02-peer-protocol.md#L172,

temporary_channel_id is required to be unique only within the same peer.

Then, this seems too restrictive:
https://github.com/rust-bitcoin/rust-lightning/blob/f975639d036b08706fc007aa327bffbc0a5f217b/src/ln/channelmanager.rs#L1265-L1267

[TheBlueMatt]

Hmm, indeed, though honestly I'd kinda like to get the spec changed for that, it seems kinda shitty that we have to make the map use a (node_id, temporary_channel_id) instead of just channel_id key and I'd be highly surprised if any clients currently violate that.

[TheBlueMatt]

Technically we violate the spec, so this should stay open, even if I'd prefer the solution be a change to the spec.

[TheBlueMatt]

Note that this should come for free with #422.