This was motivated by trying to allow for completely cleaning up
resource pools, which involved removing their use of fiberize which
currently has no destroy mechanism.
As part of this, there's a new parallelism limiter mechanism using
resource pools rather than fibers, and also a fixed size resource
pool.
The tests now drain? and destroy the resource pools to check cleaning
up.
Add the ability to specify the max number of waiters for a resource
pool, this provides a more efficient way of avoiding waiters for a
resource pool continually rising.
This commit also improves the destroy behaviour.
Mostly to no longer sleep in the main fiber. Now the main fiber just
spawns other fibers when it would previously block on put-operation
and these other fibers communicate back to the main resource pool
fiber when necessary.
This should mean that the resource pool is more responsive.
