upon requests from backends, rather than on a fixed 500msec cycle.  (There's
still throttling logic to ensure it writes no more often than once per
500msec, though.)  This should result in a significant reduction in stats file
write traffic in typical scenarios where the stats are demanded only
infrequently.

This approach also means that the former difficulty with changing
stats_temp_directory on-the-fly has gone away, so remove the caution about
that as well as the thrashing we did to minimize the trouble window.

In passing, also fix pgstat_report_stat() so that we will send a stats
message if we have function call stats but not table stats to report;
this fixes a bug in the recent patch to support function-call stats.

Martin Pihlak