Refactor coin prices calculation

[timurkhafizov]

Part of https://trello.com/c/BINUSz3X/8-refactor-portfolio-controller

Synopsis

This PR drastically increases time of /portfolio page rendering by properly utilizing Redis cached keys. In order to make it working we must populate Redis cache first. The steps which are needed to make it working:

1. Deploy app
2. In console run $redis.del($redis.keys('*_price_*'))
3. In console run Coins::SyncDailyPricesForCoinsWorker.perform_async

Comparison

1. master branch - https://apm.scoutapp.com/dev_traces/ca892158-e460-4bff-8cf6-73d9cd97c975
2. This branch - https://apm.scoutapp.com/dev_traces/9343e406-e48d-43f1-a6fb-cf0eac5f5939

Changes

• [x] Use namespaced cache keys for coin historical prices
• [x] Properly set and read price cache keys
• [x] Move price caching to a separate hourly worker
• [x] Move calculation of coin prices to a separate service object