Improve logging of external to internal IRI translation of Agent in Session

[jholleran]

If the Agent IRI has the same domain as the Base IRI there need to be a translation to an internal data IRI inside the Session before calling methods such as: https://github.com/trellis-ldp/trellis/blob/master/auth/webac/src/main/java/org/trellisldp/webac/WebAcService.java#L204

Two options that we could do here:

1. Clearly log that the internal IRI translation is needed so its easier to debug when this issue happens.
2. Trellis to internally do the translation so that external client code does not need to worry about it.