[validation.cpp] update 'cousins' during UpdateFlags

Summary:
In some obscure cases during UpdateFlags, it can happen that we update
an ancestor which itself has children that need updating.

For example, we might call reconsiderblock on a block whose parent is marked
invalid. This would remove the failed flag from said parent block, in which case
all descendants under that one ought to have their FailedParent flag revoked too.

For example consider:

O --- F --- FP* --- FP --- FP
       \
        --- FP --- F --- FP

O = okay,  F = failed, FP = failed parent, * = reconsidered block

Currently, running reconsiderblock on FP* would yield

O --- O --- O* --- O --- O
       \
        --- FP --- F --- FP

This diff would instead do:

O --- O --- O* --- O --- O
       \
        --- O --- F --- O

Note that it's OK if too many blocks get FailedParent removed, like in that
example. This is fine because FindMostWorkChain will notice this and put the
FailedParent flag back in place:

O --- O --- O* --- O --- O
       \
        --- O --- F --- FP

(and it will also find the new correct value for pindexBestInvalid)

Depends on D5041

This patch was written by @markblundeberg

Test Plan: ninja check-all

Reviewers: #bitcoin_abc, jasonbcox, markblundeberg

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4936