ref(rust): Remove strategy.close

[untitaker]

I long felt that nobody ever does anything useful in this method in Rust
or in Python. And it makes it a bit harder to think about what "state"
the strategy is in, since strategy.join() sometimes calls close() and
then can no longer be called multiple times.

[untitaker]

Also strategy.close is incorrectly implemented in rust's RunTaskInThreads. it closes the next strategy before it does its own flushing, which leads to this sequence of events on rebalancing:

1. StreamProcessor calls self.close
2. ...calls next_step.close
3. StreamProcessor calls self.join
4. ...calls next_step.submit x 100
5. ...calls next_step.join

according to the API contract, next_step could panic at step 4 because close was already called. luckily nobody ever implemented that in Rust