Simplify JobContext

[onlyann]

The JobContext class mixes fields that are intended to be used by consumers of this library while other fields are used internally. 

As part of v3, it is a good time to clean up JobContext and only expose the fields that are intended for public use.
There is also the opportunity to simplify typing by removing the task field.

[medihack]

Closed by #1213

[ewjoachim]

(as seen in #1257 (comment), maybe the task was useful after all...)

[onlyann]

(as seen in #1257 (comment), maybe the task was useful after all...)

Rather than reintroducing a circular reference with the task, how about adding useful fields directly on JobContext, such as the retry strategy instance in this case?

[ewjoachim]

I think it was pretty useful to have the task there in the end. Is there really no simple way we can make it work for typing ?