pgbench stats per script & other stuff

This patch adds per-script statistics & other improvements to pgbench

Rationale: Josh asked for the per-script stats:-)

Some restructuring is done so that all stats (-l --aggregate-interval
--progress --per-script-stats, latency & lag...) share the same structures
and functions to accumulate data. This limits a lot the growth of pgbench
from this patch (+17 lines).

In passing, remove the distinction between internal and external scripts.
Pgbench just execute scripts, some of them may be internal...

As a side effect, all scripts can be accumulated "pgbench -B -N -S -f ..."
would execute 4 scripts, 3 of which internal (tpc-b, simple-update,
select-only and another externally supplied one).

Also add a weight option to change the probability of choosing some scripts
when several are available.

Hmmm... Not sure that the --per-script-stats option is really useful. The
stats could always be shown when several scripts are executed?

sh> ./pgbench -T 3 -B -N -w 2 -S -w 7 --per-script-stats
starting vacuum...end.
transaction type: multiple scripts
scaling factor: 1
query mode: simple
number of clients: 1
number of threads: 1
duration: 3 s
number of transactions actually processed: 3192
latency average: 0.940 ms
tps = 1063.756045 (including connections establishing)
tps = 1065.412737 (excluding connections establishing)
SQL script 0: <builtin: TPC-B (sort of)>
- weight is 1
- 297 transactions (tps = 98.977301)
- latency average = 3.001 ms
- latency stddev = 1.320 ms
SQL script 1: <builtin: simple update>
- weight is 2
- 621 transactions (tps = 206.952539)
- latency average = 2.506 ms
- latency stddev = 1.194 ms
SQL script 2: <builtin: select only>
- weight is 7
- 2274 transactions (tps = 757.826205)
- latency average = 0.236 ms
- latency stddev = 0.083 ms

--
Fabien