Code review (pep8, pylint, epydoc)

- Added comments to make pylint quiet on broad-except handling in
notification methods
- Removed the _local_uid member of Herald Directory, as it was only used
in rare cases to cache the _local.uid call
Remove sort transports by number of peers in *_group methods

The transports should be sorted by number of handled peers (i.e. by
number of messages to send) instead of peers having a common transport.