Improve logging

[keskiju]

• [ ] If COMMON_LOG_FORMAT is 'text', still use the StackdriverStream, but print in human readable format (e.g. with this https://github.com/benbria/bunyan-debug-stream). On local development the human readable format is currently achieved with | bunyan -l debug on server/package.json.
• [ ] Prune useless logging (already done?)
• [ ] All API validation errors should leave an informative message to log (see issue #7) (already done?)
• [ ] Show db connection pool status on log in addition to request latency