Use an MVCC snapshot, rather than SnapshotNow, for catalog scans.

SnapshotNow scans have the undesirable property that, in the face of
concurrent updates, the scan can fail to see either the old or the new
versions of the row.  In many cases, we work around this by requiring
DDL operations to hold AccessExclusiveLock on the object being
modified; in some cases, the existing locking is inadequate and random
failures occur as a result.  This commit doesn't change anything
related to locking, but will hopefully pave the way to allowing lock
strength reductions in the future.

The major issue has held us back from making this change in the past
is that taking an MVCC snapshot is significantly more expensive than
using a static special snapshot such as SnapshotNow.  However, testing
of various worst-case scenarios reveals that this problem is not
severe except under fairly extreme workloads.  To mitigate those
problems, we avoid retaking the MVCC snapshot for each new scan;
instead, we take a new snapshot only when invalidation messages have
been processed.  The catcache machinery already requires that
invalidation messages be sent before releasing the related heavyweight
lock; else other backends might rely on locally-cached data rather
than scanning the catalog at all.  Thus, making snapshot reuse
dependent on the same guarantees shouldn't break anything that wasn't
already subtly broken.

Patch by me.  Review by Michael Paquier and Andres Freund.
pg_upgrade:  revert changing '' to ""
On the command line, GUC option strings are handled by the guc parser,
not by the shell parser, so '' is the proper way to represent a
zero-length string.  This reverts commit
3132a9b7ab3d76c15f88cfa29792fd888e7a959e.
Silence compiler warning in assertion-enabled builds.

With -Wtype-limits, gcc correctly points out that size_t can never be < 0.
Backpatch to 9.3 and 9.2. It's been like this forever, but in <= 9.1 you got
a lot other warnings with -Wtype-limits anyway (at least with my version of
gcc).

Andres Freund
Mention extra_float_digits in floating point docs

Make it easier for readers of the FP docs to find out about possibly
truncated values.

Per complaint from Tom Duffey in message
F0E0F874-C86F-48D1-AA2A-0C5365BF5118@trillitech.com

Author: Albe Laurenz
Reviewed by: Abhijit Menon-Sen
Add support for multiple kinds of external toast datums.

To that end, support tags rather than lengths for external datums.
As an example of how this can be used, add support or "indirect"
tuples which point to some externally allocated memory containing
a toast tuple.  Similar infrastructure could be used for other
purposes, including, perhaps, support for alternative compression
algorithms.

Andres Freund, reviewed by Hitoshi Harada and myself
pg_restore: Error about incompatible options

This mirrors the equivalent error cases in pg_dump.
doc: Arrange See Also links in more consistent order
Add #include to make header file independent
doc: Remove i18ngurus.com link

The web site is dead, and the Wayback Machine shows that it didn't have
much useful content before.