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To prepare for the addition of a new function allowing changing a
pending task's priority, the internal data structure is slightly
changed. The (optional) task ID is stored as part of the task entry. A
new dictionary provides a mapping from the task ID to its task entry. If
the task ID is None, the entry is not added to the map.

Task entries used to be a tuple, but since modifying the priority
requires changing an entry, they are changed to lists in this patch.
Tuple items can not be modified.

The underlying idea is from [1].

[1]:
http://docs.python.org/library/heapq.html#priority-queue-implementation-notes

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>