Move SizeOfHeapNewCid next to xl_heap_new_cid struct.

They belong together, but the xl_heap_rewrite_mapping struct was wedged
in between.
Fix bug in the new B-tree incomplete-split code.

Inserting a downlink to an internal page clears the incomplete-split flag
of the child's left sibling, so the left sibling's LSN also needs to be
updated.