ClientAwaitedAction now uses a channel to notify drops happened

[allada]

Instead of using Weak references and custom spawns, we now use
a channel to notify receivers that cleanups are needed.

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This change is 

[allada]

+@adam-singer
+@zbirenbaum

Reviewable status: 0 of 2 LGTMs obtained (waiting on @adam-singer and @zbirenbaum)

[adam-singer]

Reviewed 20 of 20 files at r1, 3 of 3 files at r2, 2 of 2 files at r3, all commit messages.
Reviewable status: 0 of 2 LGTMs obtained, and 3 discussions need to be resolved (waiting on @zbirenbaum)

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nativelink-scheduler/src/cache_lookup_scheduler.rs line 230 at r1 (raw file):

            match maybe_action_result {
                Ok(action_result) => {
                    println!("{action_result:?}");

nit: remove println

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nativelink-scheduler/src/operation_state_manager.rs line 91 at r1 (raw file):

    // /// The operation must have it's last client update before this time.
    // NOTE: NOT PART OF ANY FILTERING!
    // TODO!(cleanup)

Can we leave a note of what should be cleaned up ?

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nativelink-scheduler/src/scheduler_state/state_manager.rs line 130 at r3 (raw file):

            .expect("AwaitedAction should be present");
        awaited_action.dec_listening_clients();
        // If we failed to send it means noone is listening.

nit: noone -> none

[allada]

Reviewable status: 0 of 2 LGTMs obtained, and pending CI: vale, and 2 discussions need to be resolved (waiting on @adam-singer and @zbirenbaum)

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nativelink-scheduler/src/cache_lookup_scheduler.rs line 230 at r1 (raw file):

Previously, adam-singer (Adam Singer) wrote…

nit: remove println

Done.