Make tokio an optional dependency for core module

[sunng87]

At the moment, tokio is a required dependency even if we are using async-std. This patch make it optional to clean up dependency tree.

[bikeshedder]

tokio/sync is a required dependency. (not tokio, as explained in #88)

tokio used to be split into tokio-sync, tokio-util, tokio-io, etc. Currently it's an all-in-one crate with feature flags. If you add tokio = { version = "1", features = ["sync"] } you are only getting tokio::sync and not the rest of it. The tokio runtime is NOT included.

deadpool needs tokio::sync::Semaphore to function. That dependency is not optional.

The CI tests do pass because they are run with --all-features and therefore tokio is enabled as well.

[bikeshedder]

If you want to make tokio/sync optional you first need to create a new Semaphore implementation that supports the same features as tokio::sync::Semaphore does:

• fair
• low level (adding and removing permits without a guard)
• closing (used for shutdown purposes)

The semaphore implementation from async-lock (which is reexported by async-std) supports neither of those features. It's rather primitive to be honest.

The semaphore from futures-intrusive could be used, but that comes with even more dependencies.

tokio::sync::Semaphore is currently the only choice available that has all the required features.

[bikeshedder]

I've added tests to the CI that should cover this. If you rebase your PR to the latest version of the master branch the checks should now be failing.

[sunng87]

Understand. Thank you for clarification.