Add support for CONFIG GET

[shawwn]

On redis-cli version 6.0.6, ./benchmark.sh produces warnings like:

$ ./benchmark.sh 
pydis 1
Could not connect to Redis at 127.0.0.1:7878: Connection refused
WARN: could not fetch server CONFIG
Could not connect to Redis at 127.0.0.1:7878: setsockopt(TCP_NODELAY): Invalid argument
pydis 2
Could not connect to Redis at 127.0.0.1:7878: Connection refused
WARN: could not fetch server CONFIG
Could not connect to Redis at 127.0.0.1:7878: setsockopt(TCP_NODELAY): Invalid argument
pydis 3
Could not connect to Redis at 127.0.0.1:7878: Connection refused
WARN: could not fetch server CONFIG
Could not connect to Redis at 127.0.0.1:7878: setsockopt(TCP_NODELAY): Invalid argument
redis 1
Could not connect to Redis at 127.0.0.1:6379: Connection refused
WARN: could not fetch server CONFIG
All clients disconnected... aborting.

This PR implements support for a very basic, hardcoded server config that squelches this warning.