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_server.cpp

@@ -253,6 +253,11 @@ t_command_server::t_command_server(
     , std::bind(&t_command_parser_executor::relay_tx, &m_parser, p::_1)
     , "Relay a given transaction by its txid"
     );
+    m_command_lookup.set_handler(
+      "sync_info"
+    , std::bind(&t_command_parser_executor::sync_info, &m_parser, p::_1)
+    , "Print information about blockchain sync state"
+    );
 }
 
 bool t_command_server::process_command_str(const std::string& cmd)