[enh] Add timeout limit per request (#1640)

The new url parameter "timeout_limit" set timeout limit defined in second.
Example "timeout_limit=1.5" means the timeout limit is 1.5 seconds.

In addition, the query can start with <[number] to set the timeout limit.

For number between 0 and 99, the unit is the second :
Example: "<30 searx" means the timeout limit is 3 seconds

For number above 100, the unit is the millisecond:
Example: "<850 searx" means the timeout is 850 milliseconds.

In addition, there is a new optional setting: outgoing.max_request_timeout.
If not set, the user timeout can't go above searx configuration (as before: the max timeout of selected engine for a query).

If the value is set, the user can set a timeout between 0 and max_request_timeout using
<[number] or timeout_limit query parameter.

Related to #1077
Updated version of PR #1413 from @isj-privacore

tests/unit/test_query.py

@@ -62,3 +62,45 @@ class TestQuery(SearxTestCase):
         self.assertEquals(len(query.query_parts), 1)
         self.assertEquals(len(query.languages), 0)
         self.assertFalse(query.specific)
+
+    def test_timeout_below100(self):
+        query_text = '<3 the query'
+        query = RawTextQuery(query_text, [])
+        query.parse_query()
+
+        self.assertEquals(query.getFullQuery(), query_text)
+        self.assertEquals(len(query.query_parts), 3)
+        self.assertEquals(query.timeout_limit, 3)
+        self.assertFalse(query.specific)
+
+    def test_timeout_above100(self):
+        query_text = '<350 the query'
+        query = RawTextQuery(query_text, [])
+        query.parse_query()
+
+        self.assertEquals(query.getFullQuery(), query_text)
+        self.assertEquals(len(query.query_parts), 3)
+        self.assertEquals(query.timeout_limit, 0.35)
+        self.assertFalse(query.specific)
+
+    def test_timeout_above1000(self):
+        query_text = '<3500 the query'
+        query = RawTextQuery(query_text, [])
+        query.parse_query()
+
+        self.assertEquals(query.getFullQuery(), query_text)
+        self.assertEquals(len(query.query_parts), 3)
+        self.assertEquals(query.timeout_limit, 3.5)
+        self.assertFalse(query.specific)
+
+    def test_timeout_invalid(self):
+        # invalid number: it is not bang but it is part of the query
+        query_text = '<xxx the query'
+        query = RawTextQuery(query_text, [])
+        query.parse_query()
+
+        self.assertEquals(query.getFullQuery(), query_text)
+        self.assertEquals(len(query.query_parts), 1)
+        self.assertEquals(query.query_parts[0], query_text)
+        self.assertEquals(query.timeout_limit, None)
+        self.assertFalse(query.specific)