[release/1.4.6] eth/downloader: stream partial skeleton filling to processor

(cherry picked from commit e86619e75d)

eth/downloader/downloader_test.go

@@ -1258,6 +1258,7 @@ func testInvalidHeaderRollback(t *testing.T, protocol int, mode SyncMode) {
 	// rolled back, and also the pivot point being reverted to a non-block status.
 	tester.newPeer("block-attack", protocol, hashes, headers, blocks, receipts)
 	missing = 3*fsHeaderSafetyNet + MaxHeaderFetch + 1
+	delete(tester.peerHeaders["fast-attack"], hashes[len(hashes)-missing]) // Make sure the fast-attacker doesn't fill in
 	delete(tester.peerHeaders["block-attack"], hashes[len(hashes)-missing])
 
 	if err := tester.sync("block-attack", nil, mode); err == nil {