[PERF] Remove lambda allocation in ServiceProvider.GetService

[rynowak]

https://github.com/aspnet/DependencyInjection/blob/dev/src/Microsoft.Framework.DependencyInjection/ServiceProvider.cs#L52

This lamdba is allocating about 6mb across 10000 requests of a simple MVC app. We could easily make this 0 by turning the lambda into an instance method and caching a delegate to it.