redis.Redis.from_url(url) doesn't check if the url is valid

Before this commit: actual error are detected later when the client is actually used.
With this commit, client() makes sure to return a valid Redis client or None.

Also, the code makes sure not to log the password of the Redis URL

settings.yml:
* The default URL was unix:///usr/local/searxng-redis/run/redis.sock?db=0
* The default URL is now "false"

The default URL makes the log difficult to deal with:
if the admin didn't install a Redis instance, the logs record a false error.

It worked before because SearXNG initialized the Redis connection when the limiter started.

In this commit, SearXNG initializes Redis in searx/webapp.py
so various components can use Redis without taking care of the initialization step.

Remove the abstraction in searx.shared.SharedDict.
Implement a basic and dedicated scheduler for the checker using a Redis script.

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

The errors make pyright usage useless since a new error won't be seen [1].

[1] https://github.com/searxng/searxng/pull/1569

```
  searx/compat.py:11:27 - error: Expression of type "Type[cached_property[_T@cached_property]]" cannot be assigned to declared type "Type[cached_property]"
    "Type[cached_property[_T@cached_property]]" is incompatible with "Type[cached_property]"
    Type "Type[cached_property[_T@cached_property]]" cannot be assigned to type "Type[cached_property]" (reportGeneralTypeIssues)
  searx/utils.py:69:36 - error: Expression of type "None" cannot be assigned to parameter of type "str"
    Type "None" cannot be assigned to type "str" (reportGeneralTypeIssues)
  searx/utils.py:573:85 - error: Expression of type "None" cannot be assigned to parameter of type "int"
    Type "None" cannot be assigned to type "int" (reportGeneralTypeIssues)
  searx/webapp.py:1306:22 - error: Argument of type "str" cannot be assigned to parameter "__a" of type "BytesPath" in function "join"
    Type "str" cannot be assigned to type "BytesPath"
      "str" is incompatible with "bytes"
      "str" is incompatible with protocol "PathLike[bytes]"
        "__fspath__" is not present (reportGeneralTypeIssues)
  searx/webapp.py:1306:68 - error: Argument of type "Literal['themes']" cannot be assigned to parameter "paths" of type "BytesPath" in function "join"
    Type "Literal['themes']" cannot be assigned to type "BytesPath"
      "Literal['themes']" is incompatible with "bytes"
      "Literal['themes']" is incompatible with protocol "PathLike[bytes]"
        "__fspath__" is not present (reportGeneralTypeIssues)
  searx/webapp.py:1306:78 - error: Argument of type "str | Any | None" cannot be assigned to parameter "paths" of type "BytesPath" in function "join"
    Type "str | Any | None" cannot be assigned to type "BytesPath"
      Type "str" cannot be assigned to type "BytesPath"
        "str" is incompatible with "bytes"
        "str" is incompatible with protocol "PathLike[bytes]"
          "__fspath__" is not present (reportGeneralTypeIssues)
  searx/webapp.py:1306:85 - error: Argument of type "Literal['img']" cannot be assigned to parameter "paths" of type "BytesPath" in function "join"
    Type "Literal['img']" cannot be assigned to type "BytesPath"
      "Literal['img']" is incompatible with "bytes"
      "Literal['img']" is incompatible with protocol "PathLike[bytes]"
        "__fspath__" is not present (reportGeneralTypeIssues)
  searx/engines/mongodb.py:8:6 - warning: Import "pymongo" could not be resolved (reportMissingImports)
  searx/engines/mysql_server.py:9:8 - warning: Import "mysql.connector" could not be resolved (reportMissingImports)
  searx/engines/postgresql.py:9:8 - warning: Import "psycopg2" could not be resolved from source (reportMissingModuleSource)
  searx/engines/xpath.py:187:28 - warning: "categories" is not defined (reportUndefinedVariable)
  searx/search/__init__.py:184:82 - warning: "flask" is not defined (reportUndefinedVariable)
  searx/search/checker/background.py:19:26 - error: Type of "schedule" is partially unknown
    Type of "schedule" is "(delay: Any, func: Any, *args: Any) -> Literal[True]" (reportUnknownVariableType)
  searx/shared/__init__.py:8:12 - warning: Import "uwsgi" could not be resolved (reportMissingImports)
  searx/shared/shared_uwsgi.py:5:8 - warning: Import "uwsgi" could not be resolved (reportMissingImports)
```

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