Add support for rebinding existing Schema to messages classes

[mbrannstrom]

SchemaBuilder can generate a Schema from Java message classes, where the Schema is bound to the classes.

Add support for binding an existing Schema (e.g. received over the wire) to Java message classes. There may be discrepancies between the Java message classes and the Schema, e.g. some fields missing in the Schema and some missing (optional) fields in the message classes.

This is needed to allow for schema upgrades, when e.g. a client have message classes for protocol version 1.2 and receives schema version 1.3 over the wire (from the server).

Schema compatibility on two levels:

1. Can decode (parts of) the message.
2. Can encode at least all required parts of the message.

The first compatibility is required for all messages (groups), while the second is only required for messages sent by this party.