Using Redis in socket mode no longer works

I had a demo of ratelimit running in a Kubernetes cluster at the end of 2019. For performance reasons I had ratelimit connect to a Redis instance over a socket shared on a ConfigMap volume. Having the ratelimit -> Redis connection not use a network at all is great for performance.

Turns out https://github.com/envoyproxy/ratelimit/commit/3ec0f5f3a660ab3ce1f6e16f3823e18dd1ceebf7#diff-ab160e3a4ff5cb5fd488f666c5266fdd disabled the default Unix socket usage by hardcoding the use of TCP.

It is not possible to launch ratelimit with `REDIS_SOCKET_TYPE=unix` even though it is the 'default' value. See an example below:

```
env USE_STATSD=false LOG_LEVEL=debug REDIS_URL=/tmp/redis.sock REDIS_SOCKET_TYPE=unix RUNTIME_ROOT=examples/ratelimit/config/ RUNTIME_SUBDIRECTORY=. bin/ratelimit
WARN[0000] statsd is not in use
DEBU[0000] runtime changed. loading new snapshot at examples/ratelimit/config
DEBU[0000] runtime: processing examples/ratelimit/config
DEBU[0000] runtime: processing examples/ratelimit/config/config.yaml
DEBU[0000] runtime: adding key=config.yaml value=---
domain: mongo_cps
descriptors:
- key: database
value: users
rate_limit:
unit: second
requests_per_unit: 500

- key: database
value: default
rate_limit:
unit: second
requests_per_unit: 500
uint=false
WARN[0000] connecting to redis on /tmp/redis.sock with pool size 10
panic: dial tcp: address /tmp/redis.sock: missing port in address

goroutine 1 [running]:
github.com/envoyproxy/ratelimit/src/redis.checkError(...)
/home/moderation/Library/envoyproxy/ratelimit/src/redis/driver_impl.go:44
github.com/envoyproxy/ratelimit/src/redis.NewPoolImpl(0xca07e0, 0xc00010eb40, 0x0, 0x0, 0x0, 0xc00003a2ea, 0xf, 0xa, 0xc00011e080, 0xc000134000)
/home/moderation/Library/envoyproxy/ratelimit/src/redis/driver_impl.go:98 +0x344
github.com/envoyproxy/ratelimit/src/service_cmd/runner.(*Runner).Run(0xc000213f68)
/home/moderation/Library/envoyproxy/ratelimit/src/service_cmd/runner/runner.go:57 +0x317
main.main()
/home/moderation/Library/envoyproxy/ratelimit/src/service_cmd/main.go:7 +0x43
```
The socket type setting at https://github.com/envoyproxy/ratelimit/blob/master/src/settings/settings.go#L22 is no longer used in the code base.