pgbench - add minimal stats on initialization

Hello devs,

The attached patch adds minimal stats during the initialization phase. 
Such a feature was already submitted by Doug Rady two years ago,

     https://commitfest.postgresql.org/15/1308/

but it needed to be adapted to the -I custom initialization approach 
developed in the same CF, and it ended 'returned with feedback'.

   sh> ./pgbench -i -s 3
   dropping old tables...
   creating tables...
   generating data...
   100000 of 300000 tuples (33%) done (elapsed 0.09 s, remaining 0.18 s)
   200000 of 300000 tuples (66%) done (elapsed 0.20 s, remaining 0.10 s)
   300000 of 300000 tuples (100%) done (elapsed 0.32 s, remaining 0.00 s)
   vacuuming...
   creating primary keys...
   done in 0.68 s (drop 0.06 s, create table 0.02 s, generate 0.34 s, vacuum 0.13 s, primary keys 0.13 s).

See the durations on the last line.

The intent is to test the initialization phase more precisely, and 
possibly accelerate it. For instance, is it better to do vacuum before or 
after primary keys?

-- 
Fabien.