Stellar XLM: Technical Spec for Horizon API

XLM will define a schematized API for how Horizon fetches data from stellar-core. The new approach will remove the tight coupling between Horizon and the stellar-core database, reduce load on the stellar-core database from Horizon, allow stellar-core to update its DB schema without needing Horizon changes, and improve Horizon ingestion performance. It should also make it easier to build non-Horizon systems on top of stellar-core via a simple and stable API. This is just the planning step, implementation will happen later in 2019.