Support way to copy managed objects into a Realm.

[clementetb]

There are situations, like during a discard local client reset, that we like to copy managed or outdated objects into a Realm.

Right now the solution is to manually copy such objects, it would be convenient to the user to have a method to import such objects automatically.

[cmelchior]

We also want to implement a generic Realm.copyFromRealm, so this tracks the special use case of Client Resets where you are moving objects between versions, which could be done a lot faster in native code.