Add more comments about timelock assumptions and security model

Rename HTLC_FAIL_ANTI_REORG_DELAY to ANTI_REORG_DELAY because
we are going to rely on it also to remove bump candidates outpoint
from tracker after claim get enough depth.

Rename HTLC_FAIL_TIMEOUT_BLOCKS to LATENCY_GRACE_PERIOD_BLOCKS because
it's carrying more meaningfully that we are doing a favor to our
peer instead of ruthlessly enforcing the contract.

CLTV_EXPIRY_DELTA should be > to LATENCY_GRACE_PERIOD_BLOCKS +
+CLTV_CLAIM_BUFFER + ANTI_REORG_DELAY + LATENCY_GRACE_PERIOD_BLOCKS
When we reached height + LATENCY_GRACE_PERIOD_BLOCKS and we have pending
unsolved outbound HTLC, we fail onchain with
our local commitment tx. At this point we expect to get in chain in a
worst-case delay of CLTV_CLAIM_BUFFER. When our HTLC-timeout is
confirmed with ANTI_REORG_DELAY we may safely fail backward the
corresponding inbound output.