Move from spin to conquer-once

[ErichDonGubler]

This resolves RUSTSEC-2019-0031.

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The advisory page suggests several alternatives to the now-deprecated spin crate. The reason I picked conquer-once was because it seemed to have the most similar (and limited) API to how spin was being used -- it seemed a good small dependency to trade for.

[ErichDonGubler]

The CI failures here don't seem related, esp. since they're occurring in other PRs right now (for example, this one).

[kellpossible]

Seems like spin is being maintained again now with a new 0.6 release?

[ErichDonGubler]

@kellpossible: I've asked in upstream spin for clarification around the current maintainership of the project.

[briansmith]

Thanks for the PR. I also saw your most recent comment in the issue. I do think we should just close this. I don't know if conquer-once or spin-rs is better but also I don't want to spend a lot of time on the issue. As was noted in the issue, cargo deny is no longer warning about spin-rs so this isn't urgent. Thanks again for the PR though!