Fix AsyncConnection usage in sentinel if aio feature is not used.

[brocaar]

Hello @jaymell I believe this should fix the issue with the aio import within sentinel (#922).

[nihohit]

@brocaar how can the issue be replicated? I tried running cargo build --features sentinel and it completed succesfully.

[felipou]

I replicated it by using the redis crate in another crate, using only the sentinel feature. I don't know why the build command is not failing though.

[nihohit]

Well, it's a real shame if we can't add to the CI some way of checking whether dependencies are broken in this way.
@jaymell, any ideas?

[jaymell]

@nihohit I've seen errors like this get masked previously by the redis-test crate. If you explicitly build only the redis crate, you will see the issue, i.e. with cargo build -p redis --features sentinel.

It looks like there is a tool tokio is using called cargo-hack that can test features individually, which we can probably wire up easily in CI to do a check per dependency. Let me see if I can throw something together.

[jaymell]

Thanks for the quick turnaround on the PR. @brocaar if you rebase your PR, the checks should all pass.

[brocaar]

Thanks for the review. @jaymell I have just pushed my rebased changes.