use message-id instead of str(message) in logs

Using message-id in log messages makes it easier to identify individual
messages.

The existing string representation of a Message makes it difficult to identify a specific message.  This patch causes log messages to include the message id -- prefixed by `id:` so that it can be passed directly to `notmuch show`.  This makes it much easier to debug filter problems (since it becomes trivial to view the email associated with a particular log message).