Fix gpstart know nothing about unavailable segments.
Fix gpstart know nothing about unavailable segments.

The code snippet about `inactiveDbIds` in `_prepare_segment_start()` 
make `invalid_segs` always empty, so gpstart will think that there are no segments that are not up.
50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,adam8157/gpdb,ashwinstar/gpdb,50wu/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,xinzweb/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,jmcatamney/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,ashwinstar/gpdb