WIP adds Context support to manager in place of stop channels.

[mhrivnak]

This is backward-incompatible. The manager accepts Context instead of a stop
channel, which helps simplify cancellation workflow and is more in line with
the direction related projects are moving.

This pattern can be expanded in a similar way to controllers, the cache, and
so on if people like it.

fixes #103

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[mhrivnak]

@droot @DirectXMan12 @JoelSpeed see what you think of this approach, using a Context. If we're going to make a backward-incompatible change, this may be a good direction to go.

[DirectXMan12]

I like this pattern. Perhaps we can also have signals.SetupSignalHandler work more nicely with this?

[JoelSpeed]

This makes sense to me and seems to solve the original problem :)

[droot]

@mhrivnak Thanks for trying this approach. I like it :)

[droot]

Closing this PR since the purpose of this PR was to get a feel for the context based solution, which we achieved. The original issue is solved using inner-stop-channel. Thanks @mhrivnak for the POC.