[enh] add redis connector searx/shared/redisdb.py

Add a redis connector, the default DB connector is a socket at::

    unix:///usr/local/searxng-redis/run/redis.sock?db=0

To set up a redis instance simply use::

    $ ./manage redis.build
    $ sudo -H ./manage redis.install

A hint for developers:

To get access rights to this instance, your developer account needs to be added
to the *searxng-redis* group::

    $ sudo -H ./manage redis.addgrp "${USER}"
    # don't forget to logout & login to get member of group

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/settings_defaults.py

@@ -169,6 +169,9 @@ SCHEMA = {
         'method': SettingsValue(('POST', 'GET'), 'POST'),
         'default_http_headers': SettingsValue(dict, {}),
     },
+    'redis': {
+        'url': SettingsValue(str, 'unix:///usr/local/searxng-redis/run/redis.sock?db=0'),
+    },
     'ui': {
         'static_path': SettingsDirectoryValue(str, os.path.join(searx_dir, 'static')),
         'templates_path': SettingsDirectoryValue(str, os.path.join(searx_dir, 'templates')),