feat: Add a new cluster state "Unhealthy"

[sbernauer]

Part of #58
WIP, it still has commits from #62 until that is merged

[Techassi]

Feature overall lgtm, just a few questions and comments.

[Techassi]

note: I don't quite understand the sentence in line 15. It is either wrong, or worded weirdly.

note: markdownlint complains about the <br /> tag. I think we should remove it.

[sbernauer]

You are right, there was a typo: a9726af

[sbernauer]

TIL a backslash also works: 5e5b022

[Techassi]

note: You should add a small doc comment explaining what this method does.

[sbernauer]

Added in 373ab07

[Techassi]

question: Why exactly do we set the number of queries to zero here? Can you add a developer comment here which shortly explains it?

[sbernauer]

I added some rustdoc in 373ab07 explaining it. Is that sufficient in this case here?

[Techassi]

note: It seems like we are not actually mapping anything here. Does it maybe make more sense to use .for_each() here?

[sbernauer]

Nope, as we are using https://docs.rs/futures/latest/futures/future/fn.join_all.html here

    Checking trino-lb v0.3.2 (/home/sbernauer/stackable/trino-lb/trino-lb)
error[E0308]: mismatched types
   --> trino-lb/src/maintenance/query_count_fetcher.rs:139:54
    |
139 |                         .for_each(|(cluster, state)| self.process_cluster(cluster, state))
    |                                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found future
    |
note: calling an async function returns a future
   --> trino-lb/src/maintenance/query_count_fetcher.rs:139:54
    |
139 |                         .for_each(|(cluster, state)| self.process_cluster(cluster, state))
    |                                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: consider `await`ing on the `Future`
    |
139 |                         .for_each(|(cluster, state)| self.process_cluster(cluster, state).await)
    |                                                                                          ++++++
help: consider using a semicolon here
    |
139 |                         .for_each(|(cluster, state)| { self.process_cluster(cluster, state); })
    |                                                      +                                     +++

error[E0277]: `()` is not an iterator
   --> trino-lb/src/maintenance/query_count_fetcher.rs:135:30
    |
135 |                   let result = join_all(
    |  ______________________________^
136 | |                     self.clusters
137 | |                         .iter()
138 | |                         .zip(cluster_states)
139 | |                         .for_each(|(cluster, state)| self.process_cluster(cluster, state))
140 | |                 )
    | |_________________^ `()` is not an iterator
    |
    = help: the trait `Iterator` is not implemented for `()`
    = note: required for `()` to implement `IntoIterator`

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `trino-lb` (bin "trino-lb") due to 2 previous errors

We could do some things around futures::stream::iter(self.clusters.iter().zip(cluster_states)).for_each_concurrent(1000000, |(cluster, state)| self.process_cluster(cluster, state)).await, but
a.) that a different approach than we take everywhere else.
b.) The result.len() breaks, we need to do it differently

Not sure if that's worth it