Factorize backend sync in crypto SDK

For now each method modifying a crypto value (secret or public) is responsible for calling synchronization methods (typically updateSecretBackupOnServer). It would be better if values that need synchronization are modified through “setter“ methods which handle synchronization.

However there are cases where we want to modify some values without triggering an automatic sync, typically when the call to the server is done by the application code, for instance in account creation, hard password reset...

One beneficial side-effect of this factorization would be a class OwnerCryptoContext which code is smaller and much easier to read and understand, with most of the store logic moved to another file.