pgsql: Fix bug due to confusion about what IsMVCCSnapshot means

Fix bug due to confusion about what IsMVCCSnapshot means

In 0b96e734c59 I (Andres) relied on page_collect_tuples() being called only
with an MVCC snapshot, and added assertions to that end, but did not realize
that IsMVCCSnapshot() allows both proper MVCC snapshots and historical
snapshots, which behave quite similarly to MVCC snapshots.

Unfortunately that can lead to incorrect visibility results during logical
decoding, as a historical snapshot is interpreted as a plain MVCC
snapshot. The only reason this wasn't noticed earlier is that it's hard to
reach as most of the time there are no sequential scans during logical
decoding.

To fix the bug and avoid issues like this in the future, split
IsMVCCSnapshot() into IsMVCCSnapshot() and IsMVCCLikeSnapshot(), where now
only the latter includes historic snapshots.

One effect of this is that during logical decoding no page-at-a-time snapshots
are used, as otherwise runtime branches to handle historic snapshots would be
needed in some performance critical paths. Given how uncommon sequential scans
are during logical decoding, that seems acceptable.

Author: Antonin Houska <ah(at)cybertec(dot)at>
Reported-by: Antonin Houska <ah(at)cybertec(dot)at>
Discussion: https://postgr.es/m/61812.1770637345@localhost

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ce5d489166e5c8437aa1a35f462f67f1aeae87d4

Modified Files
--------------
src/backend/access/heap/heapam_handler.c | 2 +-
src/backend/access/index/indexam.c | 2 +-
src/backend/access/nbtree/nbtree.c | 2 +-
src/include/utils/snapmgr.h | 21 ++++++++++++++++++---
4 files changed, 21 insertions(+), 6 deletions(-)