Zero out padding bytes for memtuple.
Zero out padding bytes for memtuple.

heaptuple_form_to() zero's out the buffer, so this commit does same
for memtuple_form_to(). This is done to have zeros in padding areas
and hence help get good compression for AO tables.

This also helps to eliminate the flakiness seen in appendonly test for
compression ratio. Now given same input the compression ratio will be
same.

Discussion:
https://groups.google.com/a/greenplum.org/d/msg/gpdb-dev/1N5Qi-4WPis/EAIdwvy9CAAJ

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