Be less aggressive in outbound HTLC CLTV timeout checks

Be less aggressive in outbound HTLC CLTV timeout checks

We currently assume our counterparty is naive and misconfigured and
may force-close a channel to get an HTLC we just forwarded them.

There shouldn't be any reason to do this - we don't have any such
bug, and we shouldn't start by assuming our counterparties are
buggy. Worse, this results in refusing to forward payments today,
failing HTLCs for largely no reason.

Instead, we keep a fairly conservative check, but not one which
will fail HTLC forwarding spuriously - testing only that the HTLC
doesn't expire for a few blocks from now.

Fixes #1114.