pgsql: Fix heap_page_prune's problem with failing to send cache

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/access/heap:
        pruneheap.c (r1.7 -> r1.8)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/pruneheap.c?r1=1.7&r2=1.8)
    pgsql/src/backend/utils/cache:
        inval.c (r1.83 -> r1.84)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/inval.c?r1=1.83&r2=1.84)
    pgsql/src/include/utils:
        inval.h (r1.41 -> r1.42)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/inval.h?r1=1.41&r2=1.42)