Improve batching of Awaitable XHP
Improve batching of Awaitable XHP

If there were nested awaitable XHP elements, then the batching could be
split apart by the recursion. This was because
`__flushElementChildren()` was being called from within the loop, so
childrens’ awaiting would cause the loop to stop. Just set all children
and then recursively call `__flushElementChildren()` at the end.

fixes facebook/xhp-lib#132

facebook/xhp-lib