Cache stats in Redis to avoid db connections

[karpet]

We can leverage Redis to cache things that do not change often, like stats. This should reduce DB load and HTTP requests.

• provision Redis at Heroku
• add Docker example of running Redis locally
• refactor /api endpoints to use Redis
• refactor /stats to use Redis

[karpet]

example of my local docker-compose.yml file for postgres and redis servers:

version: '3'
services:
  redis:
    image: redis:2.8.23
    ports:
      - "6379:6379"

  ksvotes-postgres:
    image: postgres:11.7
    container_name: ksvotes-db
    ports:
      - "5432:5432"
    volumes:
      - "postgresdata:/var/lib/postgresql/data"
    environment:
      - POSTGRES_PASSWORD=postgres

volumes:
  postgresdata:

which I invoke like:

% docker-compose up -d