Add explicit VACUUM to inet test to actually do IOS

Add explicit VACUUM to inet test to actually do IOS

Some of the queries in inet test are meant to exercise Index Only Scans.
Postgres-XL was not however picking those plans due to stale stats on
the coordinator (reltuples and relpages in pg_class).

On plain PostgreSQL the tests work fine, as CREATE INDEX also updates
statistics stored in the pg_class catalog. For example this

    CREATE TABLE t (a INT);

    INSERT INTO t SELECT i FROM generate_series(1,1000) s(i);

    SELECT relpages, reltuples FROM pg_class
     WHERE relname = 't';

    CREATE INDEX ON t(a);

    SELECT relpages, reltuples FROM pg_class
     WHERE relname = 't';

will show zeroes before the CREATE INDEX command, and accurate values
after it completes.

On Postgres-XL that is not the case, and we will return zeroes even after
the CREATE INDEX command. To actually update the statistics we need to
fetch information from the datanodes the way VACUUM does it.

Fixed by adding an explicit VACUUM call right after the CREATE INDEX, to
fetch the stats from the datanodes and update the coordinator catalogs.