Replace tag indexer with event streaming API

[kostko]

The Bleve-based runtime tag indexer should be removed and replaced with an event streaming API that covers both the consensus layer and runtime events. The event sink implementations can then use something like PostgreSQL to index events.

The streaming API should support the following:

• Streaming consensus and runtime events to be indexed (separate between block and transaction events). Error handling so events can be reindexed in case of indexing errors.
• Querying for events using a general enough query abstraction.

Related issues that may be obsolete/subsumed by this are #3388 and #1674.

[kostko]

Actually this probably doesn't need any support from an Oasis Node as long as all the relevant information for a specific consensus/runtime height/round can be queried from it. What should probably be done instead is to define a standardized gRPC API and build an external indexer that supports "advanced" queries based on the index.