Improve handling of unconnecting headers

When processing a headers message that looks like a block announcement,
send peer a getheaders if the headers message won't connect.

Apply DoS points after too many consecutive unconnecting headers messages.

src/main.h

@@ -138,6 +138,9 @@ static const bool DEFAULT_FEEFILTER = true;
 /** Maximum number of headers to announce when relaying blocks with headers message.*/
 static const unsigned int MAX_BLOCKS_TO_ANNOUNCE = 8;
 
+/** Maximum number of unconnecting headers announcements before DoS score */
+static const int MAX_UNCONNECTING_HEADERS = 10;
+
 static const bool DEFAULT_PEERBLOOMFILTERS = true;
 
 struct BlockHasher