Don't delete requests if we're choked per BEP 6

This should make the expected receive chunk counts match up more correctly. It doesn't seem to affect tests at the moment, but then we don't verify the expected receive chunk counts are correct either.

peerconn.go

@@ -1040,7 +1040,9 @@ func (c *PeerConn) mainReadLoop() (err error) {
 		switch msg.Type {
 		case pp.Choke:
 			c.peerChoking = true
-			c.deleteAllRequests()
+			if !c.fastEnabled() {
+				c.deleteAllRequests()
+			}
 			// We can then reset our interest.
 			c.updateRequests()
 			c.updateExpectingChunks()