protocol: speed up sync by minimizing duplicate work

In particular, the prepare_handle_incoming_blocks call
is pretty lengthy, and entirely pointless in the common
case where several different connections will prepare
the exact same blocks.

src/cryptonote_protocol/cryptonote_protocol_handler.h

@@ -135,6 +135,7 @@ namespace cryptonote
     std::atomic<bool> m_synchronized;
     bool m_one_request = true;
     std::atomic<bool> m_stopping;
+    epee::critical_section m_sync_lock;
 
     boost::mutex m_buffer_mutex;
     double get_avg_block_size();