rpc: add a busy_syncing field to get_info

true if and pretty much only if new blocks are being added

src/rpc/core_rpc_server.cpp

@@ -499,6 +499,7 @@ namespace cryptonote
     res.update_available = restricted ? false : m_core.is_update_available();
     res.version = restricted ? "" : MONERO_VERSION_FULL;
     res.synchronized = check_core_ready();
+    res.busy_syncing = m_p2p.get_payload_object().is_busy_syncing();
 
     res.status = CORE_RPC_STATUS_OK;
     return true;