Fix gistkillitems for GiST root page

Require share-exclusive lock to set hint bits and to flush

heapam: Move logic to handle HEAP_MOVED into a helper function

Add very basic test for kill_prior_tuples

aio: Add README.md explaining higher level design

aio: Fix assertion, clarify README

aio: Fix reference to outdated name

aio: Fix possible state confusions due to interrupt processing

aio: Improve debug logging around waiting for IOs

aio: Fix crash potential for pg_aios views due to late state update

Change relpath() et al to return path by value

Change _mdfd_segpath() to return paths by value