pgbench - allow to create partitioned tables

Hello devs,

While doing some performance tests and reviewing patches, I needed to
create partitioned tables. Given the current syntax this is time
consumming.

The attached patch adds two options to create a partitioned "account"
table in pgbench.

It allows to answer quickly simple questions, eg "what is the overhead of
hash partitioning on a simple select on my laptop"? Answer:

# N=0..?
sh> pgench -i -s 1 --partition-number=$N --partition-type=hash

# then run
sh> pgench -S -M prepared -P 1 -T 10

# and look at latency:
# no parts = 0.071 ms
# 1 hash = 0.071 ms (did someone optimize this case?!)
# 2 hash ~ 0.126 ms (+ 0.055 ms)
# 50 hash ~ 0.155 ms
# 100 hash ~ 0.178 ms
# 150 hash ~ 0.232 ms
# 200 hash ~ 0.279 ms
# overhead ~ (0.050 + [0.0005-0.0008] * nparts) ms

--
Fabien.