[mod] poolrequests: for one (user request, engine) always use the same HTTPAdapter

The duckduckgo engine requires an additional request after the results have been sent.
This commit makes sure that the second request uses the same HTTPAdapter
= the same IP address, and the same proxy.

searx/poolrequests.py

@@ -1,7 +1,7 @@
 import sys
 from time import time
 from itertools import cycle
-from threading import RLock, local
+from threading import local
 
 import requests
 
@@ -88,10 +88,12 @@ class SessionSinglePool(requests.Session):
         super().__init__()
 
         # reuse the same adapters
-        with RLock():
         self.adapters.clear()
-            self.mount('https://', next(https_adapters))
-            self.mount('http://', next(http_adapters))
+
+        https_adapter = threadLocal.__dict__.setdefault('https_adapter', next(https_adapters))
+        http_adapter = threadLocal.__dict__.setdefault('http_adapter', next(http_adapters))
+        self.mount('https://', https_adapter)
+        self.mount('http://', http_adapter)
 
     def close(self):
         """Call super, but clear adapters since there are managed globaly"""