Report stats when replaying XLOG_RUNNING_XACTS

Report stats when replaying XLOG_RUNNING_XACTS

Previously stats in the startup process would only get reported during
shutdown of the startup process. It has been that way for a long time, but
became a lot more noticeable with the new pg_stat_io view, which separates out
IO done by different backend types...

While replaying after every XLOG_RUNNING_XACTS isn't the prettiest approach,
it has the advantage of being quite easy. Given that we're well past feature
freeze...

It's not a problem that we don't report stats more frequently with
wal_level=minimal, in that case stats can't be read before the stats process
has shut down.

Besides the above, this commit also changes pgstat_report_stat() to acquire
the timestamp with GetCurrentTimestamp() instead of
GetCurrentTransactionStopTimestamp().

Thanks to Melih Mutlu, Kyotaro Horiguchi for prototypes of other approaches to
solving this issue.

Reported-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Discussion: http://postgr.es/m/5315aedc-fbca-1556-c5de-dc2e00b23a14@oss.nttdata.com