pgsql: Modify the stats regression test to delay until the stats file

Log Message:
-----------
Modify the stats regression test to delay until the stats file actually
changes (with an upper limit of 30 seconds), and record the delay time in
the postmaster log.  This should give us some info about what's happening
with the intermittent stats failures in buildfarm.  After an idea of
Andrew Dunstan's.

Modified Files:
--------------
    pgsql/src/test/regress/expected:
        stats.out (r1.8 -> r1.9)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/stats.out.diff?r1=1.8&r2=1.9)
    pgsql/src/test/regress/sql:
        stats.sql (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/stats.sql.diff?r1=1.6&r2=1.7)