We want this change to avoid MVC eagerly reading the form. This is good
for general perf and also for scenarios where you want read the body
yourself (large file uploads).
We DO have scenarios where you want to configure the value providers
per-request or also to change the limits on the value providers (form) so
it's worth keeping these around on the context.
This change separates model binding into IModelBinderProvider (decides
which binder to use) and IModelBinder (does binding). The
IModelBinderFactory is a new services with coordinates the creation of
model binders.
Ryan Nowak
N. Taylor Mullen