cryptonote_protocol_handler: sync speedup

A block queue is now placed between block download and
block processing. Blocks are now requested only from one
peer (unless starved).

Includes a new sync_info coommand.

src/daemon/command_parser_executor.h

@@ -134,6 +134,8 @@ public:
   bool update(const std::vector<std::string>& args);
 
   bool relay_tx(const std::vector<std::string>& args);
+
+  bool sync_info(const std::vector<std::string>& args);
 };
 
 } // namespace daemonize