Models nullability review 2

[mtmk]

In addition to the earlier work on models review these models too:

• NATS.Client.Services.Models
• and NATS.Client.ObjectStore.Models

Workflow

1. Remove any default! assignments
2. Stick "required" on non-nullable reference types that don't have a default value
3. Add ? to all the optional reference types

see also #300
cc @mnmr