[Epoch Sync] Prevent header or block application at genesis if epoch sync is due

[robin-near]

Description

If we're waiting for epoch sync to happen, we should refuse any headers or blocks we receive. Otherwise, it will ruin the requirement that header_head is at genesis, making us unable to do epoch sync.

This should not happen normally (as long as the code isn't buggy to actually enter header or block sync when we should be doing epoch sync), but someone malicious can still send us headers or blocks on top of genesis to prevent us from epoch syncing.