This increases the CLTV_CLAIM_BUFFER constant to 18, much better
capturing how long it takes to go on chain to claim payments.
This is also more in line with other clients, and the spec, which
sets the default CLTV delay in invoices to 18.

As a side effect, we have to increase MIN_CLTV_EXPIRY_DELTA as
otherwise as are subject to an attack where someone can hold an
HTLC being forwarded long enough that we *also* close the channel
on which we received the HTLC.

Our enforced requirements for HTLC acceptance is that we have at
least HTLC_FAIL_BACK_BUFFER blocks before the HTLC expires. When we
receive an HTLC, the HTLC would be "already expired" if its
`cltv_expiry` is current-block + 1 (ie the next block could
broadcast the commitment transaction and time out the HTLC). From
there, we want an extra HTLC_FAIL_BACK_BUFFER in blocks, plus an
extra block or two to account for any differences in the view of
the current height before send or while the HTLC is transiting the
network.