[release-v2.0] spv: Accept cfilters from more peers.

If initial sync takes longer than six blocks and peers have not changed,
sync will hand at ninety-nine percent unable to get cfilters for the
last few blocks. Also accept cfilters from a peer that gave us headers
at or above the desired height in this case.

Backport of 1d9f076.

Author: JoeGruffins

spv/backend.go

@@ -52,8 +52,10 @@ func pickForGetCfilters(lastHeaderHeight int32) func(rp *p2p.RemotePeer) bool {
 		// blocks are generated while performing the sync, therefore
 		// the initial advertised peer height would be lower than the
 		// last header height.  Therefore, accept peers that are
-		// close, but not quite at the tip.
-		return rp.InitialHeight() >= lastHeaderHeight-6
+		// close, but not quite at the tip. It's possible that sync
+		// until now took longer than six blocks. In that case get
+		// cfilters from a peer that gave us headers.
+		return rp.InitialHeight() >= lastHeaderHeight-6 || rp.LastHeight() >= lastHeaderHeight
 	}
 }