CSHARP-603: Make MongoCredentialStore helper class internal and standarize on Credential (no "s") for singular and Credentials for plural.
Fix CSHARP-637: Fluent Lambda Expressions Don't Resolve Interfaces

    Accidental regression from https://github.com/mongodb/mongo-csharp-driver/commit/5a62ca86fba984803e445a1d182bc125a56c7714 caused generic interface member mappings to throw.

    Update - added code to handle non-trivial edge case of explicit interface implementations caught during code review of JIRA-637 and rebased off of latest master changes.
minor code reformatting.
code review comments.