Optimise use of IEX Cloud API

[jrhenderson1988]

Attempt to reduce our usage of the IEX Cloud API in order to stay within quota limits. Particularly when requesting OHLC data for securities, adapt API calls to only request the section of data required. Potentially look to storing segments of requested data in DynamoDB and using it as our primary data source, reverting to IEX Cloud API when we are missing data.