Shuffle configuration of table and connection config in wtperf.

I've been meaning to change the way URIs are specified in wtperf for a while - so that it's more consistent with the recommended URI format for WiredTiger (using a table: uri, and a type specifier for the underlying data source).

That led to this pull request. The most significant change is that table and connection configuration strings now get appended to the existing values. If it doesn't work that way it is necessary to specify all of the table or connection configuration options.