Fix handling of inbound uncommitted feerate updates

Fix handling of inbound uncommitted feerate updates

If we receive an update_fee but do not receive a commitment_signed,
we should not persist the pending fee update to disk or hold on to
it after our peer disconnects.

In order to make the code the most readable, we add a state enum
which matches the relevant states from InboundHTLCState, allowing
for more simple code comparison between inbound HTLC handling and
update_fee handling.