Index builder 0.0.15-alpha.1 release

[dzhelezov]

A major update for the index-builder library and the GraphQL Server

Hydra Indexer

• Hydra Indexer publishes the current head to Redis and caches metrics for quick access
• Schema update: created_at, deleted_at, updated_at, version added to the Indexer entity tables
• Added additional indices

Hydra Indexer API

• Add indexerStatus with the current indexer head
• Add substrateEventsAfter to fetch events after a given event ID with additional filters

Hydra Processor

• Hydra Processor uses a GraphQL endpoint for fetching events, extrinsics, and the current indexer head
• More efficient event fetching

[dzhelezov]

Superseded by #63