Port backend-priority feature from sms-gateway

[SeiryuZ]

Backport backend priority feature from sms gateway

[SeiryuZ]

@marsha97

[marsha97]

• [x] create model Backends without user
  • alias
  • priority
  • is_active
• [x] create cached_backend module with functions:
  • delete()
  • get() also make the test
• [x] override Backend save method, so it resets the cache, every time backend is saved
• [x] create flatten() method in model Backend + test
• [ ] create get_max_retry() and get_exceptions_to_retry()functions in settings
• [ ] update method dispatch
• [ ] create test for:
  • dispatch
  • retry
  • queued_queryset

cc: @SeiryuZ @selwin