Refactor context task to allow asynchronous dispose

[glopesdev]

The current implementation of ContextTask is blocking with respect to both register access, start and stop of acquisition and context disposal, with a significant amount of state shared across calls.

Here we propose to refactor the implementation to eliminate almost all shared state, and instead redefine the acquisition task as a proper TPL task composed of two sub-tasks, readFramesand distributeFrames. This task can be cancelled asynchronously and therefore allows implementing a non-blocking version of the Dispose method which simply triggers acquisition task cancellation and prevents its restart.

StartAsync method calls are also meant to be mutually exclusive, so care was taken to prevent acquisition tasks to be called concurrently on the same context.

Fixes #131

[jonnew]

Tested with breakout hardware and few headstages. I don't see any unexpected behavior.