Add ability to create multiple tables to wtperf.

Implemented by new table_count configuration option. Wtperf will create
table_count number of tables, and populate each with the full set of data,
then workload operations will be spread randomly over the set of tables.
Fix compiler warning re: uninitialized variable.
Merge pull request #867 from wiredtiger/wtperf-multi-table

Add ability to create multiple tables to wtperf.