size of the space of the item that should be reserved
slice to the reserved space if it was successfully reserved, else null. Returns non-null empty string if size = 0 and the item was successfully pushed.
Out: The length of the returned array slice is size unless the slice is null.
Reserves space for an item of <size> bytes on the queue but doesn't fill the content. The caller is expected to fill in the content using the returned slice.
size = 0 is allowed.