Fetcher concurrency issue

[popravich]

There is a problem with Fetcher, I don't know if its bad design or simply "ported as is" issue, anyway:
fetched_records and next_record methods use _wait_empty_future to halt until results are read,
but this would only work if you had only one consumer task running. If you'd need to run several
asyncio tasks for consuming and processing messages you'll definetly end up with
_wait_empty_future being overriden with other one from other task and thus leave first task
waiting for future's result until loop is closed.

https://github.com/aio-libs/aiokafka/blob/master/aiokafka/fetcher.py#L604
Basically there is no check if _wait_empty_future already set (not None)

[fabregas]

please see #29