adjustments to notebook app logging

Mainly add a log_request function (from nbviewer) for formatting tornado access logs.

Relevant effects:

- demote successful access logs to DEBUG
- demote kernel connect messages to DEBUG

which reduces the noise at INFO-level (default)
Merge pull request #4844 from minrk/tornado-logs

adjustments to notebook app logging

Mainly add a log_request function (from nbviewer) for formatting tornado access logs.

Relevant effects:

demote successful access logs to DEBUG
demote kernel connect messages to DEBUG
which reduces the noise at INFO-level (default)