[C#] Add cancel support for futures and streams (#1580)

* add future/stream cancel support

Clear up some debug
use same package location for codegen and runtime tests.

* feedback - remove dead code

Make sure writables are not dropped after cancellation.

* Apply suggestions from code review

Co-authored-by: Joel Dice <joel.dice@akamai.com>

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Co-authored-by: Joel Dice <joel.dice@akamai.com>

Author: yowl