Refine WorkerPool halt mechanism 
Refine WorkerPool halt mechanism 

Add number of workers 'dummy command' to the work pool when haltWork() is
called, each worker should get one and return.

Signed-off-by: Pengzhou Tang <ebfb6fa79a634ed64a59bdd78edfa125b950b4d8@pivotal.io>
kaknikhil/gpdb,cjcjameson/gpdb,cjcjameson/gpdb,kaknikhil/gpdb,lintzc/gpdb,ashwinstar/gpdb,lintzc/gpdb,kaknikhil/gpdb,janebeckman/gpdb,cjcjameson/gpdb,janebeckman/gpdb,adam8157/gpdb,rvs/gpdb,edespino/gpdb,50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,Chibin/gpdb,edespino/gpdb,rvs/gpdb,lisakowen/gpdb,50wu/gpdb,Quikling/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,xinzweb/gpdb,Chibin/gpdb,xinzweb/gpdb,lisakowen/gpdb,lintzc/gpdb,jmcatamney/gpdb,Chibin/gpdb,kaknikhil/gpdb,edespino/gpdb,kaknikhil/gpdb,Quikling/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,lintzc/gpdb,kaknikhil/gpdb,lisakowen/gpdb,janebeckman/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,lintzc/gpdb,janebeckman/gpdb,xinzweb/gpdb,rvs/gpdb,kaknikhil/gpdb,lintzc/gpdb,jmcatamney/gpdb,Quikling/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,edespino/gpdb,Chibin/gpdb,janebeckman/gpdb,lintzc/gpdb,Quikling/gpdb,50wu/gpdb,edespino/gpdb,xinzweb/gpdb,edespino/gpdb,Chibin/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,xinzweb/gpdb,edespino/gpdb,cjcjameson/gpdb,rvs/gpdb,janebeckman/gpdb,rvs/gpdb,adam8157/gpdb,edespino/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,Chibin/gpdb,adam8157/gpdb,greenplum-db/gpdb,cjcjameson/gpdb,yuanzhao/gpdb,Chibin/gpdb,greenplum-db/gpdb,Quikling/gpdb,kaknikhil/gpdb,janebeckman/gpdb,Quikling/gpdb,jmcatamney/gpdb,rvs/gpdb,cjcjameson/gpdb,rvs/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,janebeckman/gpdb,Chibin/gpdb,lintzc/gpdb,50wu/gpdb,50wu/gpdb,cjcjameson/gpdb,rvs/gpdb,cjcjameson/gpdb,Chibin/gpdb,rvs/gpdb,cjcjameson/gpdb,xinzweb/gpdb,cjcjameson/gpdb,Quikling/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,edespino/gpdb,lintzc/gpdb,lisakowen/gpdb,adam8157/gpdb,edespino/gpdb,yuanzhao/gpdb,janebeckman/gpdb,rvs/gpdb,Quikling/gpdb,greenplum-db/gpdb,50wu/gpdb,Quikling/gpdb,ashwinstar/gpdb,50wu/gpdb,ashwinstar/gpdb,50wu/gpdb,kaknikhil/gpdb,lisakowen/gpdb,yuanzhao/gpdb,Quikling/gpdb,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,yuanzhao/gpdb,Chibin/gpdb,adam8157/gpdb,janebeckman/gpdb