Indexer API: subscriptions

[dzhelezov]

Subtask of #24

The indexer should publish updates to the subscribers, including the following info:

• Current indexer head
• Events in the given block

[dzhelezov]

Subscriptions are currently blocked by goldcaddy77/warthog#422

As a workaround, the processor polls the indexer every minute to fetch the most recent head and events, so there will be some latency. Incorporated into 0.0.15-alpha.1 release: #62

Once subscriptions are enabled, the latency can be significantly reduced as the processor will be able to immediately issue an additional request based on the received subscription event. Note that the polling still must remain, as there are no guarantees that all the subscription events are delivered. At the same time the polling interval may be further increased with no effect on the processor data update latency.