chore(ci): use --feature-powerset --depth 2 in features check

[taiki-e]

This allows catching the issue like #2421.

also our CI didn't notice the failure of combining --features http1,client,server.

This can be fixed by using cargo hack --feature-powerset (all combination of features) instead of cargo hack --each-feature (each feature) in CI.
However, if we use --feature-powerset, there will be more than 700 feature combinations, so we may need to use --depth (specify a max number of simultaneous feature flags of --feature-powerset) flag to limit the number of combinations.

We have too many features to check the full powerset (776), so limit the max number of simultaneous feature flags to 2 by --depth option. I think this should be sufficient in most cases as carllerche said in taiki-e/cargo-hack#58.

Tokio has too many features to test the power set. However, most issues can be reproduced by testing at most 2 features simultaneously.

In fact, #2421 can be reproduced with a combination of two feature flags. (--features client,http1)

[taiki-e]

CI failure is unrelated and will be fixed in #2424.

[seanmonstar]

This seems right. There may be a couple extra combinations that are interesting, but perhaps they should just be explicitly added in CI instead of blowing up the depth.