pgsql: Add temporary debug logging, in 9.4 branch only.

Add temporary debug logging, in 9.4 branch only.

Commit 5ee940e1c served its purpose by demonstrating that buildfarm
member okapi is seeing some sort of locally-visible state mismanagement,
not a cross-process data visibility problem as I'd first theorized.
Put in some elog(LOG) messages in hopes of gathering more info about
exactly what's happening there.  Again, this is temporary code to be
reverted once we have buildfarm results.

Discussion: https://postgr.es/m/6744.1523833660@sss.pgh.pa.us

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e55380f3b60108d402f64131fe655b0e5ccc1f31

Modified Files
--------------
src/backend/commands/indexcmds.c |  6 ++++++
src/backend/utils/time/snapmgr.c | 14 ++++++++++++++
src/include/utils/snapmgr.h      |  2 ++
3 files changed, 22 insertions(+)