Refactor association establishment and fix #589

[naterichman]

A lot of the logic in association establishment for both client/server is repeated in the sync/async methods. This PR attempts to extract any non-io logic into separate methods that can be shared between those methods.

In addition it also reduces duplication in

• Sending/receiving PDUs
• Error management for Server/Client

And finally, it adds tests that reproduce and confirm the fix for 589.

Full list of changes

• Module changes
  • Move Error enums from Client/Server to one enum at the module level
  • Add a NegotiatedOptions struct to represent values discovered during
    association.
  • Rename get_client_pdu and get_client_pdu_async to read_pdu_from_wire
    and read_pdu_from_wire_async and move to module level
• ClientAssociation changes
  • Split non-io related logic out of client association
  • Creating the A-ASSOCIATE request and processing the response don't use IO
    so they can be refactored out of establish and re-used for the sync and async
    methods.
  • Fix Stream loss in association establishment when association acceptor sends more PDUs in quick succession #589
• ServerAssociation changes
  • Split non-io related logic out of client association
  • Processing the A-ASSOCIATE request doesn't use IO so it can be factored out
    and re-used for the async implementation
  • Use read_pdu_from_wire and read_pdu_from_wire_async where possible
  • Fix Stream loss in association establishment when association acceptor sends more PDUs in quick succession #589
• Tests
  • Add methods on Client to test fix for Stream loss in association establishment when association acceptor sends more PDUs in quick succession #589
  • Add methods on Server to test fix for Stream loss in association establishment when association acceptor sends more PDUs in quick succession #589

[Enet4]

The small API changes in dicom_ul are acceptable. The new tests are sound and they are all passing. Building and running the various DIMSE tools they seem to be working as intended. And the rest of the code changes look OK at a few glances.

Great! With this one merged it should now be easier to wrap up for 0.9.0. Thanks once again! 👍