SCQ: Add combining arena

[ben-manes]

Decide on which approaches and implement. The arena should be lazily initialized and grow on contention to reduce the footprint in uncontested usages.

Optimistic w/ combiner mode
Once a producer can combine, it can no longer offer back to the arena on a CAS failure to the queue. The advantage is that no garbage is created as the arena holds only unlinked nodes.

Optimistic with dynamic mode
A producer can pivot between a combiner or transferrer on each failed CAS loop to the queue. A batch insert requires knowing the both the first and last nodes forming a linked list to be appended to the queue. The arena slot must hold both references.

Linearizable
In this mode, a producer that transferred its node must wait it to be added before completing the call. This requires a wrapper with a flag that the waiter spins on and, upon insertion, the combiner sets.