Stop using deprecated parts of metrics-lib.

We're currently using two metrics-lib types that have been deprecated:

• org.torproject.descriptor.impl.DescriptorParseException and
• org.torproject.descriptor.ExitListEntry.

It's easy to avoid using those parts and move to non-deprecated types. That would also remove the last dependency on a type in the impl package. Will provide a branch shortly (once I have a ticket number).