Fork choice fix issue 1574

[djrtwo]

Fixes #1574

Ensure justified checkpoint in store updates when finalized checkpoint updates if either of the following:

• new justified is higher epoch than previous in store
• previous justified checkpoint in store is not in the chain with the new finalized checkpoint

Built off of branch epochs-in-fork-choice to use the fixed get_ancestor logic. Will switch to v010x once #1579 is merged

[djrtwo]

cc @paulhauner

[protolambda]

LGTM, and not nice use of get_ancestor for checking the chain. Should be a simple O(1) check with the proto-array implementations of prysm and lighthouse, and close to that with the old stateful approach of lodestar.