Cleanup interaction of DbSet and DbContextConfiguration

This is part of Issue #641 which is about cleaning up the use of DbContextConfiguration. DbContextConfiguration is intended to help resolve dynamic services--that is, services for which the actual type/instance of service to use depends on the current context configuration. However, it has gradually spread throughout the code as a general purpose service locator. This causes dependencies to be hidden and creates a lot of coupling to DbContextConfiguration throughout the code, so I am making a set of changes to help fix this.

In this case the DbSet needs to have access to DbContextConfiguration, through the context, because sets are created eagerly before the scoped service provider is created, but are then initialized lazily in the scope the first time they are used.
Making Ignore behave the same independently of the order relative to other configuration.

If a property or entity was added and ignored explicitly an exception will be thrown.
If a property or entity was added and ignored through conventions or data annotations the property will be present on the entity.
If a property is ignored and removed after conventions or data annotations added keys, foreign keys or indexes referencing it they will be removed as well.
The same applies to entity types and foreign keys referencing them.
Commands: Block Apply-Migration on Store Apps
Removing Microsoft.AspNet.Diagnostics.Entity from this repo
There is a corresponding pull request to add it to the Diagnostics repo