Performance improvements for the Orthanc/Sonador Plugin

• Implement models for series, study, and patient in SQLAlchemy to allow for rapid search and lookup of DICOM resources. (Models exist alongside tables in the PostgreSQL database used by Orthanc.)
• Classes for managing event scheduling and triggers from a single location SonadorServerManager.
  • Recurring (timer based) tasks are registered using register_recurring_task and change events are registered using register_serverchange_callback.
  • Events can be triggered outside of Orthanc's callback system by calling trigger_serverchange_callback.
  • Internal views are included which allow for the events to be triggered via API endpoints.
  • Added Lua callbacks to trigger on update and on delete events, which are not available as part of the Python API.
• Created base web views which can be used for handling complex REST API interactions.
  • Based on Django class based views with clear interface: __init__ -> setup -> handle method get to allow for shared logic and workflow.
  • Implemented multiple web views to handle caching logic as REST endpoints.
    • Drop/re-initialize cache tables.
    • Views to allow for launching background processes re-populate cache tables after clearing them.
    • Views to enable indexing of single models as-needed/on-demand.
• Module implementing background tasks/indexing methods to execute logic in event driven and REST endpoint code in a uniform fashion.
• C-FIND methods for DICOM interactions which utilize the cache for finding resources.