task iter does not consume in done call. fixes #6447

[tanmaykm]

This changes the iterator for Task to consume only in start and next calls.
The istaskstarted check in start is to allow it to be called multiple times from isempty.

[JeffBezanson]

This would be a nice change, but similar to the solutions proposed in #6125 it has the problem that the task runs twice before the consumer sees the first value, which is unexpected control flow.

I'm not sure isempty is meaningful for tasks. Should its value change over time, or should it mean "did/does this task produce any values?"

[micklat]

How about raising an error in isempty(t::Task)?

[tanmaykm]

Yes, this too does not prevent the producer from running one step ahead. Having nextval and nextstate seems to be the right thing to do?

[JeffBezanson]

Yes, in this case you really want nextval and nextstate. Unfortunately there are other problems with iterators that are not fully solved by that change, but perhaps we should still consider it.