Addition of a cache layer between the APIs and Vitess

[frouioui]

This PR contains the changes required by #38. A Redis Cluster is being implemented. Allowing the APIs to cache the request that are being made to Vitess.

[frouioui]

Some research about how to trace spans with Jaeger all the way to Redis has been done. The package https://godoc.org/github.com/opencensus-integrations/redis seems to contain a nice interface to send contexts.

Moreover, go-redis packages seem to handle cluster, cache, and open tracing natively. The following repo is a library for using Redis cache: https://github.com/go-redis/cache, and https://github.com/go-redis/redis is the usual Redis go package. The library official website shows how to integrate open tracing.

[frouioui]

The cache implementation made in 2e00f6e and e9ad379 allows a great decrease of the overall query time. Cache TTL was setup to 1 minute.

As we can see, the first trace does not contain cache. We cached Vitess' response, making the second span go much quicker.

Tracing is not yet implemented all the way to Redis.

[frouioui]

As #41 mentions, tracing all the way to Redis nodes wont be done in this PR.

A more optimal way to use Redis can be done in the redis.go file of both API's server package. A set and get function could take an interface.