script: add ability to exit thread after script call

This allows the request and response hooks to cause the thread to cease
sending new requests, useful when we would like to end our run without
respect to time.

The request function exits by returning a nil value; the response function
signals an exit with a boolean true/false return.
wrk: add bufsize to longopts
wrk: Don't require duration if script is present

If a script is present, durations are not required since scripts can cause
threads to finish. This allows us to run tests without worrying whether they
might complete within some certain timeframe (and also enforce that a
particular run completed)