log the handler and params at the beginning of the request

In verbose logging, write a logging callback proc so you can log
the handler and params as soon as you know them.  This logs them
with the method and path info which makes the most logical sense.
Plus you don't get request-related logs broken up by other handler-
related logs.

The summary logging's callback proc is a no-op since these logs
aren't used in summary logging.

We still need to set the handler class and params in the `env`
b/c summary logging needs access to them.

@jcredding ready for review.