Track the best parked chain tip.

Summary:
This ensures we keep track of the best parked chain, so we can reconsider it when the time comes.

Depends on D1945

Test Plan: Unfortunately, I'm not sure how to test this.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1942