This PR introduces a fundamental breaking change to the structure of ABCI response and tx tags and the way they're processed. Namely, the SDK can support more complex and aggregated events for distribution and slashing. In addition, block responses can include duplicate keys in events.

• Implement new Event type. An event has a type and a list of KV pairs (ie. list-of-lists). Typical events may look like:

"rewards": [{"amount": "5000uatom", "validator": "...", "recipient": "..."}]
"sender": [{"address": "...", "balance": "100uatom"}]

The events are indexed by {even.type}.{even.attribute[i].key}/.... In this case a client would subscribe or query for rewards.recipient='...'

• ABCI response types and related types now include Events []Event instead of Tags []cmn.KVPair.
• PubSub logic now publishes/matches against map[string][]string instead of map[string]string to support duplicate keys in response events (from Subscribing to tx event works for multiple kvpairs using same key #1385). A match is successful if the value is found in the slice of strings.

closes: #1859
closes: #2905

• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG_PENDING.md