Change variable nomenclature in chan_utils

Change variable nomenclature in chan_utils

Variables should be named according to the script semantic which is
an invariant with regards to generating a local or remote commitment
transaction.

I.e a broadcaster_htlc_key will always guard a HTLC to the party able
to broadcast the computed transactions whereas countersignatory_htlc_key
will guard HTLC to a countersignatory of the commitment transaction.