Aggregate packages with differing, but similar, locktimes

Aggregate packages with differing, but similar, locktimes

In the previous commit we started aggregating claims much more
aggressively but only did so when timelocks on HTLC timeout claims
were identical. While there's nothing we can do to change this for
HTLC-Timeout claims (on our commitment transactions), on
counterparty commitment transactions there's no such restirction.
However, we do need to avoid aggregating different claims with
drastically different locktimes to avoid delaying claims too long,
and thus introduce a new constant, `CLTV_DIFFERENCE_BATCH_CLAIM`,
to limit this.

This new batching has security implications as we may delay
claiming an HTLC for up to that many blocks, and thus we make the
constant public, update the `cltv_expiry_delta` docs, and update
some constant security checks.