Do not update peer information if ancestor search is in progress (#12631)

* Do not update peer information if ancestor search is in progress

If block announcement is received from a peer while ancestor search
for that same peer is still in progress, do not update the peer's best
hash and best number as that causes the ancestor search to yield
different information from what was expected and can cause, for example,
a fork of lower height not be be downloaded.

* Block until peers are in sync