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.

for reviewers: should we use it to add timeout support in dev::tokio functions?

May invalidate #12

[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?