Fix Redis client and type errors introduced in #24285

[VyrCossont]

Three things:

• The result of sscan_each is an iterator of single IDs, not an iterator of ID batches, and fails trying to each_slice a string in ids if run with a non-empty set of any kind of ID.
• Fixing that triggers an exception because redis can't be used inside a pipelined block (see below)
• Once that's fixed, running with a non-empty set of status IDs where the status contains a hashtag will trigger an exception but in a different place:

Pipelining commands on a Redis instance is deprecated and will be removed in Redis 5.0.0.

redis.pipelined do
  redis.get("key")
end

should be replaced by

redis.pipelined do |pipeline|
  pipeline.get("key")
end
(called from /Users/user/Projects/mastodon/app/workers/scheduler/indexing_scheduler.rb:18:in `block (3 levels) in perform'}

or

(called from /Users/user/Projects/mastodon/app/models/trends/history.rb:18:in `block in accounts'}

I think what's going on here is that this setup means that the tag indexing queue is also non-empty, and the tag index contains trend history, and calculating trend history involves another Redis call, which cannot be nested in a pipelined block. Moving the type.import! outside the pipelined block fixes this.

[nschonni]

@mjankowski was one of your Spec PRs adding coverage for this area?

Edit: rebased the CodeCov PR and it doesn't look like it's currently covered https://app.codecov.io/gh/mastodon/mastodon/blob/jest-coverage/app/workers/scheduler/indexing_scheduler.rb

[VyrCossont]

Deeper test coverage for search stuff would be great. Even if it's not covered in that PR, maybe there are other indexing or Redis-related tests? I'll have a look through it.