Minimize the race condition in BackoffSweeper()
Minimize the race condition in BackoffSweeper()

There is a long-standing race condition in BackoffSweeper() which
triggers an error and then triggers another assertion failure for
not reset sweeperInProgress to false.

This commit doesn't resolve the race condition fundamentally with
lock or other implementation, because the whole backoff mechanism
did not ask for accurate control, so skipping some sweeps should
be fine so far. We also downgrade the log level to DEBUG because
a restart of sweeper backend is unnecessary.

50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,Chibin/gpdb,xinzweb/gpdb,Chibin/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,Chibin/gpdb,adam8157/gpdb,adam8157/gpdb,ashwinstar/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,Chibin/gpdb,Chibin/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,Chibin/gpdb,greenplum-db/gpdb,Chibin/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,xinzweb/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,xinzweb/gpdb,ashwinstar/gpdb,lisakowen/gpdb,greenplum-db/gpdb,Chibin/gpdb,jmcatamney/gpdb,lisakowen/gpdb,Chibin/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,Chibin/gpdb,jmcatamney/gpdb