Allow tuple visibility checks without hint-bit, maintenance

Hi,

Table AMs that store heap-format tuples on pages managed by another WAL
scheme (generic WAL, for example) can't tolerate
HeapTupleSatisfiesVisibility() and friends opportunistically writing
hint bits to the buffer: an unlogged write between two WAL operations
invalidates the before-image a later generic-WAL delta is computed
against, and standby replay ends up with a corrupt page.

The attached patch adds HeapTupleSatisfiesVisibilityNoHints() and
HeapTupleSatisfiesUpdateNoHints(), thin wrappers that return the same
verdict without touching the page. The sentinel that makes this work
(NoHintBitsBuffer) stays private to heapam_visibility.c.

Since NoHintBitsBuffer is negative, it passes BufferIsLocal(), so any
code reached from the wrappers that uses the buffer for something
besides hint bits needs to know about it. The one such case is
SNAPSHOT_HISTORIC_MVCC, which needs the buffer to recover the tuple's
relfilelocator; HeapTupleSatisfiesVisibilityNoHints() rejects that
snapshot type outright rather than let the sentinel reach it. In
practice this shouldn't fire: that snapshot type is only used for
logical decoding's catalog lookups, always against pg_catalog, which
is always heap.

(Thanks to Euler Taveira, who helped me with this, particularly with
criticizing an earlier and more invasive proposal.)

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com