Add experimental Redis caching

CHANGELOG.md

cmd/media_repo/main.go

cmd/media_repo/reloads.go

common/config/conf_min_shared.go

common/config/models_domain.go

common/config/watch.go

common/globals/reload.go

config.sample.yaml

docs/config.md

docs/redis.md

go.mod

go.sum

internal_cache/media_cache.go

redis_cache/redis.go