Fix holding cell freeing in case we fail to add some HTLC

Fix holding cell freeing in case we fail to add some HTLC

Previously, if we went to free the holding cell HTLC updates, and
adding one failed as we hit our outbound HTLC limit (or in-flight
value limit), we would not send a commitment_signed, leaving us in
an invalid state. We first fix that bug, and then refuse to add
things to our holding cell once we reach our limits considering the
holding cell, as we shouldn't have multiple commitment dance rounds
worth of HTLCs in the holding cell anyway.