[columnar] Port in citus freezing code for vacuum

[JerrySievert]

fixes #192 by porting in fix from citus citusdata/citus#5962

I was not able to get TAP tests working correctly in our source tree, so did not include the TAP test, so here are the steps to reproduce (taken from citusdata/citus#5958):

postgresql.conf

 shared_preload_libraries=citus
 shared_preload_libraries='citus'
 vacuum_freeze_min_age = 50000
 vacuum_freeze_table_age = 50000
 synchronous_commit = off
 fsync = off

psql:

CREATE EXTENSION columnar;
CREATE TABLE test_row(i int);
INSERT INTO test_row VALUES (1);
CREATE TABLE test_col(i int) USING columnar;
INSERT INTO test_col VALUES (1);

bash:

(for i in `seq 1 200000`; do echo "UPDATE test_row SET i = i + 1;"; done) | ./bin/psql -q -X postgres

psql:

select datname, relname, age(datfrozenxid) datage, age(relfrozenxid) as relage from pg_database, pg_class where datname='postgres' and (relname='test_row' or relname='test_col');
vacuum freeze;
select datname, relname, age(datfrozenxid) datage, age(relfrozenxid) as relage from pg_database, pg_class where datname='postgres' and (relname='test_row' or relname='test_col');