Change 'This promise has already been verified' message from verbose to ...

The agent today prints a ton of these types of messages.

2013-05-28T11:20:43-0700  verbose: This promise has already been verified

This makes no sense without context. However, adding stack path, producing

2013-05-28T11:20:43-0700  verbose: /main/methods/'any': promise already verified

also provides almost no value because this message is hit whenever a promise lock is found to be valid, i.e. when a promise does not need to be evaluated anymore. This happens a lot in an agent run and produces too much output to be useful.

This fix simply moves this message to log level debug.