DicomWebClient should be more explicit about the serviceUrlPrefix

[dzelemba]

Sometimes this value contains just the base healthcare URL (e.g. https://healthcare.googleapis.com/v1 for the export adapter), sometimes it also includes the datastore path (https://healthcare.googleapis.com/v1/project/..../dicomStores/blah/dicomWeb) and sometimes it's the full STOW URL. This makes the code hard to read and understand.

[danielbeaudreau]

Specifically what we can do here to make the dicomweb client always consume the dicomweb address:

• Truncate "/studies" from the legacy stow path, and pass it in as the dicomweb path, so that the dicomweb client can now treat the two as the same.
• For the export adapter, only initialize a new dicomweb client when we have the complete address from the pubsub message + healthcare API flag.