introduce logging framework.

Added:
  Request.Logger
  NewLogger(*Request)
  DataBag

  they return Logger interface object that supports logging text or data.
  and made `SessionData` to represent arbitrary user data, which was renamed
  `DataBag` so that users can use arbitary log data for session and logging.

Removed:

  Request#Trace(string, interface...)
  Request#EnableTrace
  debug.log(string, interface...)
  SessionData
LogSinkManager concurrency tests were done.