Changes to handling partial responses

Context: paritytech#493 (comment)

Problem: When downloading from a fork, the peer may send a partial response
that extends the current chain. Even though the response starts at a block
number after the current best number, the parent block from the fork is not
yet downloaded/imported. We currently try to import the partial response
in this scenario, which causes import to fail as parent is not found in the
backend. This causes sync to be aborted/restarted which may not converge.

Fix: when we receive a partial response in this scenario, hold on to the
partial blocks and try to download the missing blocks. And import when the
full range is downloaded.

substrate/client/network/sync/src/extra_requests.rs

@@ -343,11 +343,12 @@ impl<'a, B: BlockT> Matcher<'a, B> {
 #[cfg(test)]
 mod tests {
 	use super::*;
-	use crate::PeerSync;
+	use crate::{PeerDownloadState, PeerSync};
 	use quickcheck::{Arbitrary, Gen, QuickCheck};
 	use sp_blockchain::Error as ClientError;
 	use sp_test_primitives::{Block, BlockNumber, Hash};
 	use std::collections::{HashMap, HashSet};
+	use std::ops::Range;
 
 	#[test]
 	fn requests_are_processed_in_order() {
@@ -542,7 +543,13 @@ mod tests {
 			let s = match u8::arbitrary(g) % 4 {
 				0 => PeerSyncState::Available,
 				// TODO: 1 => PeerSyncState::AncestorSearch(g.gen(), AncestorSearchState<B>),
-				1 => PeerSyncState::DownloadingNew(BlockNumber::arbitrary(g)),
+				1 => PeerSyncState::DownloadingNew(PeerDownloadState::new(
+					BlockNumber::arbitrary(g),
+					Range {
+						start: BlockNumber::arbitrary(g),
+						end: BlockNumber::arbitrary(g),
+					}
+				)),
 				2 => PeerSyncState::DownloadingStale(Hash::random()),
 				_ => PeerSyncState::DownloadingJustification(Hash::random()),
 			};