Parallel append

[alimanfoo]

Currently the append method on the Array class obtains a lock on the array, which means all append operations are forced to be sequential. An alternative approach would be to implement append by locking the array in order to resize, but then locking only at the chunk level when setting data to be appended into the newly extended region. This would allow multiple append operations to be working in parallel, because they would be writing into non-overlapping regions of the array, while still maintaining the same append semantics.

[alimanfoo]

Maybe have a method extend() which returns the new shape. If locking is handled within extend then there may be no need to keep the lock during the rest of append?