Permit printing out individual response times

[Baron Schwartz]

I'd like an option to specify a file to which each individual response time will be printed in the oltp test. The format should be something like this:

<timestamp.microseconds><TAB><responsetime.microseconds>

This will permit me to analyze how uniform the server's response times are. It would be useful for this to work with both the "prepare" and the "run" stages.