Move HTLC interception decisions to forward_htlcs callsites

In the next commit we'll substantially expand the types of HTLCs
which can be intercepted. In order to do so, we want to make
forwarding decisions with access to the (specified) destination
channel. Sadly, this isn't available in `forward_htlcs`, so here we
move interception decisions out of `forward_htlcs` and into
`process_pending_update_add_htlcs` and `handle_release_held_htlc`.

Note that we do not handle HTLC interception when forwarding an
HTLC which was decoded in LDK versions prior to 0.2, which is noted
in a suggested release note. This is due to a gap where such HTLC
might have had its routing decision made already and be waiting
for an interception decision in `forward_htlcs`, but now we will
only make an interception decision when decoding the onion.

Author: TheBlueMatt