Clarify 'Client' naming

[John Diamond]

• rename Client to Core
• rename Deepstream to Client

I'm also considering moving all core-related classes into a separate (or nested) namespace e.g. deepstream::core
This would help encapsulate core functionality from users, since library users are likely to do something like using namespace deepstream, but shouldn't then have access to all the core classes like (current) Client since this could be confusing.