Add anchor output when we have HTLCs or a to_local output

This is the first step in implementing anchor outputs - namely
adding the anchor outputs themselves. We don't ever spend them,
and this commit doesn't update other commitent transaction scripts
with the new CSV timelocks.

This commit temporarily disables the
ln::channel::tests::outbound_commitment_test test, as we don't have
test vectors for anchor-outputs-without-updated-scripts upstream.