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/rpc_command_executor.h

@@ -155,6 +155,8 @@ public:
   bool update(const std::string &command);
 
   bool relay_tx(const std::string &txid);
+
+  bool sync_info();
 };
 
 } // namespace daemonize