- implement shared_redis.RedisCacheSharedDict to store key/value pairs in the
redis DB. The default key_prefix is 'SearXNG_SharedDict'.
- simplify searx.shared and import RedisCacheSharedDict, set
key_prefix to 'SearXNG_storage'
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>