gpstop: Recognize downed segments before exiting
gpstop: Recognize downed segments before exiting

Run a distributed query across all segments to force FTS to detect and mark all
downed segments.

Author: Nadeem Ghani <6210ba2d096145a21f72d2cc5992a921fd4dd884@pivotal.io>
Author: Marbin Tan <fe9db60e23a06449b90b19d66b721708068089ad@pivotal.io>
Author: Shoaib Lari <ad045e028025f2970ed8506438710c2da1df61d5@pivotal.io>
Author: C.J. Jameson <ec1b14366327ff10b752f9848cd5b8311e885007@pivotal.io>

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