Improvements and fixes for the common load-data x-test benchmarks

Could you please rewrite the doc? I find it quite difficult to understand. I don't like the fact that usage of transactions is influenced by "transactionSize" attribute. IMO it should be set to "1" by default and appropriate exception should be thrown if the value is zero or negative & transactions are enabled. I'm wondering whether we can restrict the values in xsd (especially for numeric fields) - I don't mean it as replacement for code-level checks, but it would make the configuration process more transparent.