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.