runqueue: Change buildable/running lists to sets

runqueue: Change buildable/running lists to sets

Using positions in lists for flags is an odd choice and makes the code
hard to maintain. Maintaining a list is slow since list searches are
slow (watch bitbake -n slow massively with it) but we can use a set()
instead.

This patch uses python sets to maintain the lists of tasks in each state
and this prepares for changing the task IDs from being integers.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>