Add temporary debug logging, in 9.4 branch only.

Commit 5ee940e1 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