MODE-2294: Make server side classes serializable

Adds serialization to the server side classes.
I don't see how `LRepository`, with all that it entails, can be serializable. You can serialize the repository name, since that's durable, and you should be able to reconstruct an `LRepository` from the name. Even if you make it serializable and the implementation marks everything as transient, when the implementation object is deserialized it won't have a `JcrRepository` reference and can't reconstruct anything. IOW, won't it be useless?