rewrite RelpFrame to object-oriented structure

[kortemik]

Description
current implementation relies on external control.

perhaps proper one would include RelpParser within the rxFrame and the ByteBuffer representation in txFrame and possibly in rxFrame too?

[kortemik]

remember to provide stub implementation as well for non-frames.

[kortemik]

refactor RelpConnection into an interface and RelpConnectionImpl

[kortemik]

refactor static method authenticatedContext to non-static