Rewrite port database handling for target mode.

Previous implementation was too fragile to initiator parameters changes.
In case of port role change it could not survive different handle assigned
to the same initiator by firmware, even though initiator was logged out.
The new implementation should be more resillient to this kind of problems,
trying to work in any situation and only warn user about suspisious events.

MFC after: 1 week
Sponsored by: iXsystems, Inc.