Infrastructure: Allow only one database provider per service provider

[ajcvickers]

In EF Core 1.0 there can be multiple database providers registered in the same service provider (D.I. container). However, only one can be used with any given DbContext instance. This works by defining a set of provider services and having service resolution go through a scoped indirection based on the current context so that the correct service is returned for the provider currently being used.

This was important in pre-release versions of EF where there was no real distinction between the external application service provider and the internal service provider used by EF. This meant that there was no way to use ASP.NET Core in the normal way if the application made use of two different database providers. With the switch to EF managing its own service provider be default, it now means that using ASP.NET Core in the normal way will result in two internal service providers being created--one for each database provider.

Therefore, now that we have made the internal service provider the default experience, we can switch to a simpler model where only one database provider is allowed in the service provider. This means that:

• There is no indirection mechanism for provider services, which means that:
  • Some services that had to be scoped because they varied per provider can now be singleton
  • A provider can replace any service, not just a "provider service"
  • There is no interface/base class for providers, so providers must rely more on documentation to know which services to implement
  • There is no mechanism to inject services into providers in patch releases, so we will need a different mechanism for that
• Some caching will be duplicated for applications that do use multiple providers