srm: tidy up toString methods

The SRM provides string representation of the current status of requests.  The formatting is rather
verbose and, for container requests, somewhat difficult to follow.

This patch tidies up the output by:
   o  removing user-priority (it isn't used anywhere),
   o  only print request priority if has a non-default value,
   o  include relative time information for creation and expiry dates,
   o  remove error message as it is simply the last line of the state
      history, which is printed anyway,
   o  fix indentation so relationship between file-requests inside a
      container request is more obvious,
   o  tidy up history printing,
   o  provide concised information about remove source and destination
      SURLs and/or TURLs for CopyFileRequest objects.

Target: master