Prepare scheduler config & move owner of notify task change owner

[allada]

It is easier to do this in one PR than in two because they are dependent
on each other.

1. Moves where notifications happen in the scheduler to happen instead
   in the underlying AwaitedActionDb.
2. Prepare the config changes (non-breaking).

towards #359

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This change is 

[allada]

+@aaronmondal (please only review r1:r2), this PR is dependent on https://reviewable.io/reviews/TraceMachina/nativelink/1305

Reviewable status: 0 of 1 LGTMs obtained, and 0 of 6 files reviewed, and pending CI: pre-commit-checks (waiting on @aaronmondal)

[allada]

Reviewable status: 0 of 1 LGTMs obtained, and 0 of 6 files reviewed, and pending CI: Analyze (javascript-typescript), Analyze (python), Local / ubuntu-22.04, NativeLink.com Cloud / Remote Cache (Legacy Dockerfile Test), Publish image, Publish nativelink-worker-init, Publish nativelink-worker-lre-cc, Remote / large-ubuntu-22.04, asan / ubuntu-22.04, docker-compose-compiles-nativelink (22.04), integration-tests (22.04), macos-13, pre-commit-checks, ubuntu-20.04 / stable, ubuntu-22.04, ubuntu-22.04 / stable, windows-2022 / stable (waiting on @aaronmondal)

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nativelink-scheduler/src/default_scheduler_factory.rs line 92 at r2 (raw file):

        .unwrap_or(&ExperimentalSimpleSchedulerBackend::memory)
    {
        ExperimentalSimpleSchedulerBackend::memory => {

Note: We will soon be adding :redis here, so it was restructured this way to support the changes about to come.

[allada]

or: +@adam-singer (if you want to do these two)

Reviewable status: 0 of 2 LGTMs obtained, and 0 of 6 files reviewed, and pending CI: Analyze (javascript-typescript), Analyze (python), Local / ubuntu-22.04, NativeLink.com Cloud / Remote Cache (Legacy Dockerfile Test), Publish image, Publish nativelink-worker-init, Publish nativelink-worker-lre-cc, Remote / large-ubuntu-22.04, asan / ubuntu-22.04, docker-compose-compiles-nativelink (22.04), integration-tests (22.04), macos-13, pre-commit-checks, ubuntu-20.04 / stable, ubuntu-22.04, ubuntu-22.04 / stable, windows-2022 / stable (waiting on @aaronmondal and @adam-singer)

[adam-singer]

Reviewed 6 of 6 files at r2, all commit messages.
Reviewable status: 1 of 2 LGTMs obtained, and all files reviewed (waiting on @aaronmondal)

[adam-singer]

Reviewable status: 1 of 2 LGTMs obtained, and all files reviewed (waiting on @aaronmondal)

[allada]

-@aaronmondal

Reviewable status: complete! 1 of 1 LGTMs obtained, and all files reviewed