Apply all the clang-tidy performance-* fixes

src/xmpp/xmpp_component.cpp

@@ -445,7 +445,7 @@ void XmppComponent::send_muc_leave(const std::string& muc_name, const std::strin
     presence["to"] = jid_to;
     presence["from"] = muc_name + "@" + this->served_hostname + "/" + nick;
     presence["type"] = "unavailable";
-    const std::string message_str = std::get<0>(message);
+    const std::string& message_str = std::get<0>(message);
     XmlSubNode x(presence, "x");
     x["xmlns"] = MUC_USER_NS;
     if (self)

src/xmpp/xmpp_stanza.cpp

@@ -52,7 +52,7 @@ XmlNode::XmlNode(const std::string& name, XmlNode* parent):
   parent(parent)
 {
   // split the namespace and the name
-  auto n = name.rfind(":");
+  auto n = name.rfind(':');
   if (n == std::string::npos)
     this->name = name;
   else