Wait for the futures in the given sequence to complete.
Args:
fs: The sequence of Futures (possibly created by different
Executors) to wait upon.
timeout: The maximum number of seconds to wait. If None, then
there is no limit on the wait time.
return_when: Indicates when this function should return.
The options are:
FIRST_COMPLETED - Return when any future finishes or is
cancelled.
FIRST_EXCEPTION - Return when any future finishes by raising an
exception. If no future raises an exception
then it is equivalent to ALL_COMPLETED.
ALL_COMPLETED - Return when all futures finish or are
cancelled.
Returns:
A named 2-tuple of sets. The first set, named 'done', contains the
futures that completed (is finished or cancelled) before the wait
completed. The second set, named 'not_done', contains uncompleted
futures. Duplicate futures given to *fs* are removed and will be
returned only once.