[mod] redislib - optimize LUA script registration.

Instead of register the LUA script on each call, the LUA script is registered
once and the `Script` object is cached in global storage.

[1] https://github.com/searxng/searxng/pull/1047#discussion_r884143305



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