Implement automatic caching of API calls

[karmacoma]

We should cache the results of API calls using the Redis key/value store, with a TTL corresponding with the 10 second block time. This would save repeated round trips to the database for the same dataset.

The feature should be optional, dependent on the availability of a Redis server.

[karmacoma]

API endpoints should be conditionally cached, therefore we need some granular control. The solution should be independent of express.js with flexibility to serve both http and websocket protocols.

[karmacoma]

We should include functional tests to ensure results are cached appropriately.

[karmacoma]

Redis server connection details could be stored along the existing db connection details: https://github.com/LiskHQ/lisk/blob/development/config.json#L11

[karmacoma]

Documentation would need to be updated to reflect an optional Redis installation.