Refactor incoming HTLC accept checks out from Channel::update_add_htlc

Refactor incoming HTLC accept checks out from Channel::update_add_htlc

In the future, we plan to completely remove
`decode_update_add_htlc_onion` and replace it with a batched variant.
This refactor, while improving readability in its current form, does not
feature any functional changes and allows us to reuse the incoming HTLC
acceptance checks in the batched variant.