Suppress attempts to report dropped tables to the stats collector from a

Suppress attempts to report dropped tables to the stats collector from a
startup or recovery process.  Since such a process isn't a real backend,
pgstat.c gets confused.  This accounts for recent reports of strange
"invalid server process ID -1" log messages during crash recovery.
There isn't any point in attempting to make the report, since we'll discard
stats in such scenarios anyhow.