[enh] Add Server-Timing header (#1637)

Server Timing specification: https://www.w3.org/TR/server-timing/

In the browser Dev Tools, focus on the main request, there are the responses per engine in the Timing tab.

searx/results.py

@@ -136,6 +136,7 @@ class ResultContainer(object):
         self._ordered = False
         self.paging = False
         self.unresponsive_engines = set()
+        self.timings = []
 
     def extend(self, engine_name, results):
         for result in list(results):
@@ -319,3 +320,13 @@ class ResultContainer(object):
 
     def add_unresponsive_engine(self, engine_error):
         self.unresponsive_engines.add(engine_error)
+
+    def add_timing(self, engine_name, engine_time, page_load_time):
+        self.timings.append({
+            'engine': engines[engine_name].shortcut,
+            'total': engine_time,
+            'load': page_load_time
+        })
+
+    def get_timings(self):
+        return self.timings