Rename 'post' to 'create' to avoid confusion.

(Both 'create' and 'update' are achieved via HTTP POST operations.)
Rename Mock* to Memory* (there's nothing fake about it).
Move YouDo.DB.Mock to YouDo.DB.Memory
Use Versioned to hold the old version ID in update values.
Unify implementations of in-memory stores of Youdos and Users.