Fix hang on /indexes and /stats routes

[dureuill]

Pull Request

Related issue

Fixes #4218

Context

• A previous fix added a field to the IndexScheduler to memorize the currently_updating_index, so that accessing it through the search would return the handle without trying to open it. This resolved a hang on the search, but /indexes and /stats routes hang when search requests on multiple indexes are running #4218 reported further hangs on the /indexes and /stats routes
• These routes were shunting the IndexScheduler and using internal IndexMapper logic to access the indexes, again trying to reopen the updating index.

What does this PR do?

• Moves the logic relative to the currently_updating_index from the IndexScheduler to the IndexMapper, so that any index request to the IndexMapper can benefit from it.

Test

1. Follow reproducer from /indexes and /stats routes hang when search requests on multiple indexes are running #4218
2. Before this PR, notice a hang on /stats and /indexes, but not on /indexes/<updating_index>/search
3. After this PR, notice no hang on either of /stats, /indexes or /indexes/<updating_index>/search

[Kerollmops]

This PR looks good to me, especially because you just moved stuff from one place to another 👍
bors merge

[meili-bors]

Build succeeded:

• Run Clippy
• Run Rustfmt
• Run tests in debug
• Tests on macos-12
• Tests on ubuntu-18.04
• Tests on windows-2022