Questioning some of the changes made with the move of the spec from lightning repository

[dhk2]

#510

Removed pubkey, seconds_back, sender_key, sig_fields, signature fields because no app is using them (Podverse sends the pubkey field but only puts the value “podverse-pubkey" in it)
Removed value_msat because that information is in the invoice anyways

seconds_back has been supported by the PeerTube plugin since at least march, and is becoming more relevant as developers are discussing ways to support small splits during streaming without being eaten up by fees in per minute sending. Unsure how much research was done with other apps to determine support

The signature field has already been requested to be put back by @daveajones
#521