adding (client side) support for Contains operator in query. CR: Andrew
Query perf tests (with EF6 comparison)
Changes:
* Removing previous 'QueryExecutionPerf' tests that were ported from EF6 test suite
* Replacing with Query tests that better represent recommended coding patterns (new tests cover the same general patterns that were covered by previous tests)
* Adding EF6 equivalent of the new tests (for comparison)
* Moved the Customer/Orders/Products model to a shared location since it seems this simple model will be helpful between many tests without needing to change the model. Each set of tests still create their own database with a set of expected data that is tailored to that set of tests.

I considered sharing the POCO classes between EF6/7 tests but felt that makes the tests a little different if the EF6 ones are accessing entity types from a different assembly. We could look at having a shared assembly in the future... but doesn't seem worth it for 4 classes.
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)