Add basic support for config set and get

[fluffy88]

Adding support for CONFIG GET x and CONFIG SET x y where x is the full configuration parameter name.
This implementation lacks support for the glob-style patterns which Redis allows.

[inponomarev]

Superseded by #772, which adds thin CONFIG GET/CONFIG SET support (and more). The approach here — a separate key→value namespace that round-trips — is the same idea; #772 additionally fits the current @RedisCommand architecture, wires up the behavioural parameters (lua-time-limit, proto-max-bulk-len), and adds mock-vs-real comparison tests. Closing in its favour. Thanks for the original contribution!