Add Task.Options with an autoCancelableRunLoops property

[alexandru]

The Task run-loop needs to be customizable with a Task.Options case class that gets instantiated on runAsync to a Task.defaultOptions and then passed along in task implementations.

Task then can provide a helper such as executeWithOptions(f: Options => Options), such that we can then do:

source.executeWithOptions(_.enableAutoCancelableRunLoops)

The problem we are trying to solve is that there are certain instances in which Task.flatMap loops are better off not being automatically cancelable. Such a use-case has been discovered in the monix-kafka project, where we could get rid of synchronize on cancelation if flatMap-loops wouldn't automatically cancel.

Of course, in that case it is debatable whether getting rid of synchronization buys us anything. But it's certainly a valid use-case showing how in certain cases we are forced to synchronize and trigger an immediate cancelation precisely because later we might not get the chance.

Having Task.flatMap loops be auto-cancelable by default is also error-prone. It would be better if Task.flatMap wouldn't be auto-cancelable by default, with an option to turn this behavior on where needed.