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_protocol/cryptonote_protocol_handler.h

@@ -138,6 +138,7 @@ namespace cryptonote
     void drop_connection(cryptonote_connection_context &context, bool add_fail, bool flush_all_spans);
     bool kick_idle_peers();
     bool check_standby_peers();
+    bool update_sync_search();
     int try_add_next_blocks(cryptonote_connection_context &context);
     void notify_new_stripe(cryptonote_connection_context &context, uint32_t stripe);
     void skip_unneeded_hashes(cryptonote_connection_context& context, bool check_block_queue) const;
@@ -153,6 +154,7 @@ namespace cryptonote
     block_queue m_block_queue;
     epee::math_helper::once_a_time_seconds<30> m_idle_peer_kicker;
     epee::math_helper::once_a_time_milliseconds<100> m_standby_checker;
+    epee::math_helper::once_a_time_seconds<101> m_sync_search_checker;
     std::atomic<unsigned int> m_max_out_peers;
     tools::PerformanceTimer m_sync_timer, m_add_timer;
     uint64_t m_last_add_end_time;