Use COPY FREEZE in pgbench for faster benchmark table population.

Use COPY FREEZE in pgbench for faster benchmark table population.

While populating the pgbench_accounts table, plain COPY was
unconditionally used. By changing it to COPY FREEZE, the time for
VACUUM is significantly reduced, thus the total time of "pgbench -i"
is also reduced. This only happens if pgbench runs against PostgreSQL
14 or later because COPY FREEZE in previous versions of PostgreSQL does
not bring the benefit. Also if partitioning is used, COPY FREEZE
cannot be used. In this case plain COPY will be used too.

Author: Tatsuo Ishii
Discussion: https://postgr.es/m/20210308.143907.2014279678657453983.t-ishii@gmail.com
Reviewed-by: Fabien COELHO, Laurenz Albe, Peter Geoghegan, Dean Rasheed