Add cancellation support to session::tokio functions

[fjarri]

This should allow users to impose an external timeout (or to cancel the session loop manually, e.g. on keyboard interrupt).

Also fixed some error processing lapses in par_run_session() where closing of the incoming message channel (or internal channels) was silently ignored.

The timeout support (#12) is still under consideration. The cancellation takes care of the global timeout, or any other cases where the execution needs to be terminated gracefully, but we may still need some partial cutoff times for specific rounds (not clear at this point what the API would be).

[fjarri]

@peg FYI

[coveralls]

Pull Request Test Coverage Report for Build 14842568648

Details

• 37 of 44 (84.09%) changed or added relevant lines in 2 files are covered.
• 12 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.4%) to 72.26%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
manul/src/session/tokio.rs	33	40	82.5%

Files with Coverage Reduction	New Missed Lines	%
manul/src/session/tokio.rs	12	82.97%

Totals
Change from base Build 14842544562:	-0.4%
Covered Lines:	2149
Relevant Lines:	2974

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💛 - Coveralls

[dvdplm]

I am no expert on tokio but afaik using cancellation tokens like this is idiomatic and the code looks good to me.

As for the question I need to research it a bit more; I honestly don't know what the idiomatic pattern is for deadlines/timeouts. It sounds plausible that cancellation tokens are part of the right answer, but it also seems like there'd a higher level API that we should be using?