Slightly reorganize struct SnapshotData.
Slightly reorganize struct SnapshotData.

This squeezes out a bunch of alignment padding, reducing the size
from 72 to 56 bytes on my machine.  At least in my testing, this
didn't produce any measurable performance improvement, but the space
savings seem like enough justification.

Andres Freund

arcivanov/postgres-xl,Postgres-XL/Postgres-XL,techdragon/Postgres-XL,zeroae/postgres-xl,snaga/postgres-xl,Postgres-XL/Postgres-XL,snaga/postgres-xl,tpostgres-projects/tPostgres,50wu/gpdb,postmind-net/postgres-xl,yazun/postgres-xl,oberstet/postgres-xl,jmcatamney/gpdb,tpostgres-projects/tPostgres,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,ashwinstar/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,postmind-net/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,Postgres-XL/Postgres-XL,kmjungersen/PostgresXL,oberstet/postgres-xl,xinzweb/gpdb,ashwinstar/gpdb,adam8157/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,xinzweb/gpdb,arcivanov/postgres-xl,Postgres-XL/Postgres-XL,kmjungersen/PostgresXL,oberstet/postgres-xl,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,zeroae/postgres-xl,tpostgres-projects/tPostgres,pavanvd/postgres-xl,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,zeroae/postgres-xl,lisakowen/gpdb,kmjungersen/PostgresXL,adam8157/gpdb,snaga/postgres-xl,ovr/postgres-xl,postmind-net/postgres-xl,greenplum-db/gpdb,techdragon/Postgres-XL,zeroae/postgres-xl,yazun/postgres-xl,ovr/postgres-xl,xinzweb/gpdb,ashwinstar/gpdb,snaga/postgres-xl,ashwinstar/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,kmjungersen/PostgresXL,arcivanov/postgres-xl,lisakowen/gpdb,arcivanov/postgres-xl,snaga/postgres-xl,lisakowen/gpdb,postmind-net/postgres-xl,yazun/postgres-xl,jmcatamney/gpdb,ovr/postgres-xl,yazun/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,ovr/postgres-xl,xinzweb/gpdb,ovr/postgres-xl,arcivanov/postgres-xl,tpostgres-projects/tPostgres,jmcatamney/gpdb,zeroae/postgres-xl,postmind-net/postgres-xl,techdragon/Postgres-XL,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,techdragon/Postgres-XL,greenplum-db/gpdb,oberstet/postgres-xl,50wu/gpdb,pavanvd/postgres-xl,arcivanov/postgres-xl,yazun/postgres-xl,Postgres-XL/Postgres-XL,pavanvd/postgres-xl,adam8157/gpdb,tpostgres-projects/tPostgres,techdragon/Postgres-XL,xinzweb/gpdb,kmjungersen/PostgresXL,jmcatamney/gpdb,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,ashwinstar/gpdb