Create immutable EvaluationContext implementation

[toddbaert]

Currently, we have a single implementation of EvaluationContext: MutableContext. An immutable implementation would be ideal and should probably be used for the few cases we instantiate an EvaluationContext internally (within the SDK code).

• create new ImmutableContext (take a look at the context in C#)
• use this immutable implementation anywhere the SDK currently does new MutableContext().