Optimize DistributedLogControlLock in DistributedLog_AdvanceOldestXmin
Optimize DistributedLogControlLock in DistributedLog_AdvanceOldestXmin

DistributedLog_AdvanceOldestXmin is a hot function which hold DistributedLogControlLock
in LW_EXCLUSIVE mode during the whole lifecycle.
This lock is used to protected:
1. DistributedLogShared->oldestXmin
2. DistributedLogCtl->shared->page_buffer

The first one could be optimized using atomic operation.
The second one could be optmized using LW_SHARED mode.


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