Make ILazyLoader disposable

[ajcvickers]

Because currently it relies on the context.IsDisposed flag, but this could given the wrong result in cases whether context pooling is being used.

[ajcvickers]

This is fixed, but it doesn't help the pooling case since ILazyLoader is tied to EF's context scope, and that scope is not disposed when pooling is used. Discussed in triage and decided that this is okay because it requires entities to live beyond then end of the conceptual life of the context, use lazy loading, and use pooling all together, and even then the entities will probably just work if the context is still around. The workaround if needed is to explicitly detach entities before disposing the context.