Describe QE writers, readers and shared snapshot.
Describe QE writers, readers and shared snapshot.

Initial information collected based on current reading of code, some old docs
and issues. Lot more information will be added to the same to add as more
insights are developed, speficially about locking in QE writer and reader model.

ashwinstar/gpdb,janebeckman/gpdb,lintzc/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,kaknikhil/gpdb,50wu/gpdb,yuanzhao/gpdb,cjcjameson/gpdb,janebeckman/gpdb,adam8157/gpdb,greenplum-db/gpdb,janebeckman/gpdb,Quikling/gpdb,Chibin/gpdb,greenplum-db/gpdb,Chibin/gpdb,kaknikhil/gpdb,50wu/gpdb,ashwinstar/gpdb,janebeckman/gpdb,cjcjameson/gpdb,ashwinstar/gpdb,yuanzhao/gpdb,kaknikhil/gpdb,Quikling/gpdb,yuanzhao/gpdb,lisakowen/gpdb,adam8157/gpdb,lintzc/gpdb,50wu/gpdb,edespino/gpdb,janebeckman/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,cjcjameson/gpdb,lintzc/gpdb,greenplum-db/gpdb,edespino/gpdb,janebeckman/gpdb,yuanzhao/gpdb,adam8157/gpdb,xinzweb/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,xinzweb/gpdb,Chibin/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,xinzweb/gpdb,jmcatamney/gpdb,cjcjameson/gpdb,yuanzhao/gpdb,xinzweb/gpdb,lintzc/gpdb,rvs/gpdb,lisakowen/gpdb,janebeckman/gpdb,lintzc/gpdb,greenplum-db/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,janebeckman/gpdb,lisakowen/gpdb,xinzweb/gpdb,edespino/gpdb,lisakowen/gpdb,Chibin/gpdb,adam8157/gpdb,Chibin/gpdb,cjcjameson/gpdb,lisakowen/gpdb,Quikling/gpdb,adam8157/gpdb,adam8157/gpdb,edespino/gpdb,janebeckman/gpdb,lisakowen/gpdb,xinzweb/gpdb,Quikling/gpdb,jmcatamney/gpdb,kaknikhil/gpdb,Quikling/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,jmcatamney/gpdb,rvs/gpdb,cjcjameson/gpdb,rvs/gpdb,rvs/gpdb,yuanzhao/gpdb,kaknikhil/gpdb,xinzweb/gpdb,edespino/gpdb,edespino/gpdb,rvs/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,cjcjameson/gpdb,lisakowen/gpdb,50wu/gpdb,Chibin/gpdb,edespino/gpdb,lintzc/gpdb,lintzc/gpdb,rvs/gpdb,rvs/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,Quikling/gpdb,yuanzhao/gpdb,cjcjameson/gpdb,lintzc/gpdb,Quikling/gpdb,ashwinstar/gpdb,Quikling/gpdb,Chibin/gpdb,rvs/gpdb,lintzc/gpdb,cjcjameson/gpdb,Chibin/gpdb,kaknikhil/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,edespino/gpdb,rvs/gpdb,greenplum-db/gpdb,Chibin/gpdb,Chibin/gpdb,adam8157/gpdb,yuanzhao/gpdb,Quikling/gpdb,yuanzhao/gpdb,rvs/gpdb,edespino/gpdb,kaknikhil/gpdb,cjcjameson/gpdb,edespino/gpdb,janebeckman/gpdb,50wu/gpdb,Quikling/gpdb