Add ability for wtperf to run with multiple databases in different threads.

This can be handy when testing shared cache behavior or generating different
I/O workloads.
Merge branch 'config-parse-api' into wtperf-multiple-databases
Merge branch 'config-parse-api' into wtperf-multiple-databases

Conflicts:
	bench/wtperf/wtperf.c
	bench/wtperf/wtperf.h
Fixes to wtperf as per review comments.
Merge pull request #875 from wiredtiger/wtperf-multiple-databases

Add ability for wtperf to run with multiple databases in different threa...