Deprecates use of Task in ConfigKey

[aledsage]

Also avoid StackOverflowError in ExecutionContext.getImmediately(task)
When a non-BasicTask is passed in.

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The motivation for deprecating setting a ConfigKey as a Task is that we really shouldn't be doing it! It can get very messed up if one tries to getImmediately or get non-blocking (e.g. by a Transformer that is trying to evaluate its result when triggered). That can leave the task in a cancelled state, so all subsequent calls to task.get() (i.e. all calls to config().get(mykey)) will fail!

Instead, we should be using TaskFactory (which is how the DSL evaluation works).

It's good to deprecate this before a major release of 1.0.0.

It would be good to add an overloaded method for setting config to set(ConfigKey<T>, TaskFactory<T>) so that such usage doesn't involve ugly casting to work around the strongly typed generics. We can do that as a stage two, separate from this PR.

[ahgittin]

Looks good. Merging.

Agree re set(ConfigKey<T>, TaskFactory<T>). Would also be nice to warn but similarly we can do that in a separate PR, once we've removed other uses. In particular Java calls to DependentConfiguration eg attributeWhenReady will make Task instances so that is an obvious item to fix.

There is related work in #853 which should be built upon as this continues.