Deprecate defaultStore located at `Route.store`

[runspired]

See (private) docs for this intimate API that often trolls users of ember-engines and ember-data.

https://emberjs.com/api/ember/3.4/classes/Route/methods/store?anchor=store&show=inherited%2Cprivate

Motivations

• address learning concerns around where ember ends and ember-data begins, specifically:
  • clarify that Route.model() is not named model "because it returns ember-data models"
  • clarify that Route.model() is not 1:1 with an ember-data model and remove confusion about whether routes are allowed to have more than one model
  • remove confusion around folks thinking that routes (and dynamic params in routes) must be 1:1 with API endpoints
  • squash bugs where routes have multiple dynamic params attempt to rely on store.find
  • squash bugs where dynamic params cannot have the same name
  • demystify how the model() hook works and demystify how to request data with ember-data
  • allow ember-data to drop the long vestigial store.find method
  • remove confusion around why store is available but not the right store in engines when folks forget to wire up store
  • remove a common reason developers believe ember-data is required
  • remove a common reason developers believe ember-data is ember
• enable ember-data to kill the eager initializer and eager instance-initializer necessary to override defaultStore (prevents tree shaking, is a perf hit when apps don't need ember-data to render the entry route, and limits ember-data's ability to evolve the Store's APIs)
• move to a pattern of explicit store injection, instead of magical injection on routes and controllers
• enable folks to write unit and integration tests that register the store without needing to first unregister the store due to the eager initializer ( this is a silent and very annoying bug to encounter )
• prepare for a world in which partitioning data can be a first class route concern
• prepare for a world in which request management is first class instead of spooky action at a distance
• prepare for a world in which ember-data may be adopted incrementally
• trim the default size of ember, which includes significant infrastructure to support defaultStore

For folks that want this magical behavior, an optional addon that reopens route with either a "DefaultStore" mixin or an "ember-data route store" mixin would be more in keeping with being leaner and more explicit.

[runspired]

@rwjblue had done some exploration on deprecating this, unsure where it stalled but this prevents ember-data from cleaning up various initializer patterns which hinder the ability for folks to write tests that register a store without first unregistering the store service.

[rwjblue]

Just saw the default store mentioned in a video linked in the most recent Ember Weekly: https://youtu.be/EcKaDu0xo_A

[NullVoxPopuli]

Related:

• https://github.com/emberjs/rfcs/issues/556
• https://github.com/emberjs/rfcs/pull/557

[runspired]

this was covered by https://github.com/emberjs/rfcs/pull/955