Spread payment requests to subdirs

Currently Electrum stores all payment requests in main requests
directory. It's not going to be efficient when we have thousands of
thousands of payment requests. This patch spreads that files across
two level of subdirectories.

lib/websockets.py

@@ -63,7 +63,7 @@ class WsClientThread(util.DaemonThread):
     def make_request(self, request_id):
         # read json file
         rdir = self.config.get('requests_dir')
-        n = os.path.join(rdir, request_id + '.json')
+        n = os.path.join(rdir, 'req', request_id[0], request_id[1], request_id, request_id + '.json')
         with open(n) as f:
             s = f.read()
         d = json.loads(s)