LastCommonAncestor: use skiplist when available

Summary:
We can take advantage of the fact that skip lists use identical heights
on both chains. This function was added back in 2014 (341735eb8f) and
for some reason didn't use the skip lists introduced a few weeks prior
(c9a0918330f31dcb1d5e86b56af63c3f521d3cf2).

Test Plan: ninja check

Reviewers: jasonbcox, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5067