Fix heap_page_prune's problem with failing to send cache invalidation

Fix heap_page_prune's problem with failing to send cache invalidation
messages if the calling transaction aborts later on.  Collapsing out line
pointer redirects is a done deal as soon as we complete the page update,
so syscache *must* be notified even if the VACUUM FULL as a whole doesn't
complete.  To fix, add some functionality to inval.c to allow the pending
inval messages to be sent immediately while heap_page_prune is still
running.  The implementation is a bit chintzy: it will only work in the
context of VACUUM FULL.  But that's all we need now, and it can always be
extended later if needed.  Per my trouble report of a week ago.