In checkpoint, move the check for in-progress xacts out of critical section.
In checkpoint, move the check for in-progress xacts out of critical section.

GetVirtualXIDsDelayingChkpt calls palloc, which isn't safe in a critical
section. I thought I covered this case with the exemption for the
checkpointer, but CreateCheckPoint is also called from the startup process.

50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,adam8157/gpdb,Postgres-XL/Postgres-XL,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,jmcatamney/gpdb,oberstet/postgres-xl,adam8157/gpdb,lisakowen/gpdb,pavanvd/postgres-xl,zeroae/postgres-xl,techdragon/Postgres-XL,pavanvd/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,ovr/postgres-xl,greenplum-db/gpdb,yazun/postgres-xl,ashwinstar/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,Postgres-XL/Postgres-XL,adam8157/gpdb,jmcatamney/gpdb,zeroae/postgres-xl,Postgres-XL/Postgres-XL,xinzweb/gpdb,yazun/postgres-xl,jmcatamney/gpdb,ovr/postgres-xl,ashwinstar/gpdb,xinzweb/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,zeroae/postgres-xl,techdragon/Postgres-XL,techdragon/Postgres-XL,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,ovr/postgres-xl,jmcatamney/gpdb,ashwinstar/gpdb,pavanvd/postgres-xl,techdragon/Postgres-XL,pavanvd/postgres-xl,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,ovr/postgres-xl,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,ovr/postgres-xl,50wu/gpdb,techdragon/Postgres-XL,ashwinstar/gpdb,xinzweb/gpdb,zeroae/postgres-xl,lisakowen/gpdb,lisakowen/gpdb,yazun/postgres-xl,lisakowen/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,50wu/gpdb,oberstet/postgres-xl,pavanvd/postgres-xl,yazun/postgres-xl,jmcatamney/gpdb,zeroae/postgres-xl