APY async requests make page laggy

[EdwardMan917]

Problem:
Each pool requires a number of async call to contract to get data for APY calculation. Frontend resources get eaten up every time the APYs are being updated.

Proposals:

1. Store constant value from contract to redux to avoid unnecessary updates. Eg Cakes Per Block from Pancake master chef contract.
2. A lean backend to update and aggregate APYs regularly. The DApp get only a list of APYs via the api call. (Restful or WebSocket).
3. Contract provides APY.