feature: djangoldp activity cache

Calum Mackervoyrequested to merge
266-activity-cache into master

Closes #266 (closed)

What?

  • Adds a setting STORE_ACTIVITIES which accepts "VERBOSE", "ERROR"  or None
  • The current behaviour (of saving every activity) can be achieved by setting VERBOSE, and is useful in debugging
  • The functionality of preventing the sending of unnecessary activities is achieved by using an in-memory cache instead of with expensive database lookups

Why?

  • Improves performances of the activity queue by avoiding expensive database lookups
  • Reduces database size dramatically (we can clear the existing activity tables)
  • Removes concerns about threadsafety in the activity queue
  • Likely to improve reactivity for applications using federation like Hubl

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