Add verbose backend status helper

This allows the same verbose information to be logged in the cases where
it is of use. Mostly this information can be used to figure out why a
certain session was closed.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -301,6 +301,19 @@ private:
             });
     }
 
+    std::string get_verbose_status()
+    {
+        std::string status;
+
+        for (const auto& a : m_backends)
+        {
+            status += "\n";
+            status += a->get_verbose_status();
+        }
+
+        return status;
+    }
+
     inline bool is_large_query(GWBUF* buf)
     {
         uint32_t buflen = gwbuf_length(buf);