ChangeTracker perf tests (with EF6 comparison)

Changes:
* Removing previous 'StateManagerPerf' and 'DbContextPerf' tests that were ported from EF6 test suite
* Replacing with 'ChangeTracker' tests that better represent recommended coding patterns (new tests cover the same general patterns that were covered by previous tests)
 * There is one test that is removed that doesn't have a similar scenario in the new tests (DbContext_query_on_existing_context_with_threads). Discussed with @divega and @anpete and agreed this isn't a scenario we should be super concerned about. Multi-threading on the same context really only happens in async scenarios and we should have some tests that make use of async instead.
* Adding EF6 equivalent of the new tests (for comparison)