cryptonote_protocol_handler: search for syncing peers in "cruise mode"

When all our outgoing peer slots are filled, we cycle one peer at
a time looking for syncing peers until we have at least two such
peers. This brings two advantages:

- Peers without incoming connections will find more syncing peers
that before, thereby strengthening network decentralization

- Peers will have more resistance to isolation attacks, as they
are more likely to find a "good" peer than they were before

src/cryptonote_config.h

@@ -114,6 +114,7 @@
 #define P2P_DEFAULT_HANDSHAKE_INVOKE_TIMEOUT            5000       //5 seconds
 #define P2P_DEFAULT_WHITELIST_CONNECTIONS_PERCENT       70
 #define P2P_DEFAULT_ANCHOR_CONNECTIONS_COUNT            2
+#define P2P_DEFAULT_SYNC_SEARCH_CONNECTIONS_COUNT       2
 #define P2P_DEFAULT_LIMIT_RATE_UP                       2048       // kB/s
 #define P2P_DEFAULT_LIMIT_RATE_DOWN                     8192       // kB/s