* Refactor signature of Application.CheckTx

* Refactor signature of Application.DeliverTx

* Refactor example variable names for clarity and consistency

* Rename method variables for consistency

* Rename method variables for consistency

* add a changelog entry

* update docs

## PR

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 #1385). A match is successful if the value is found in the slice of strings.

closes: #1859
closes: #2905

## Commits:

* Implement Event ABCI type and updates responses to use events

* Update messages_test.go

* Update kvstore.go

* Update event_bus.go

* Update subscription.go

* Update pubsub.go

* Update kvstore.go

* Update query logic to handle slice of strings in events

* Update Empty#Matches and unit tests

* Update pubsub logic

* Update EventBus#Publish

* Update kv tx indexer

* Update godocs

* Update ResultEvent to use slice of strings; update RPC

* Update more tests

* Update abci.md

* Check for key in validateAndStringifyEvents

* Fix KV indexer to skip empty keys

* Fix linting errors

* Update CHANGELOG_PENDING.md

* Update docs/spec/abci/abci.md

Co-Authored-By: Federico Kunze <31522760+fedekunze@users.noreply.github.com>

* Update abci/types/types.proto

Co-Authored-By: Ethan Buchman <ethan@coinculture.info>

* Update docs/spec/abci/abci.md

Co-Authored-By: Ethan Buchman <ethan@coinculture.info>

* Update libs/pubsub/query/query.go

Co-Authored-By: Ethan Buchman <ethan@coinculture.info>

* Update match function to match if ANY value matches

* Implement TestSubscribeDuplicateKeys

* Update TestMatches to include multi-key test cases

* Update events.go

* Update Query interface godoc

* Update match godoc

* Add godoc for matchValue

* DRY-up tx indexing

* Return error from PublishWithEvents in EventBus#Publish

* Update PublishEventNewBlockHeader to return an error

* Fix build

* Update events doc in ABCI

* Update ABCI events godoc

* Implement TestEventBusPublishEventTxDuplicateKeys

* Update TestSubscribeDuplicateKeys to be table-driven

* Remove mod file

* Remove markdown from events godoc

* Implement TestTxSearchDeprecatedIndexing test

* abci: update RequestInfo for versions

* abci: update ResponseInfo for versions

* compile fix

* fix test

* software_version -> version

* comment fix

* update spec

* add test

* comments and fix test

* first commit

finalize rebase

add protoc_merkle to Makefile

* in progress

* fix kvstore

* fix tests

* remove iavl dependency

* fix tx_test

* fix test_abci_cli

fix test_apps

* fix test_apps

* fix test_cover

* rm rebase residue

* address comment in progress

* finalize rebase

* Make mempool aware of MaxGas requirement

* update spec

* Add tests

* Switch GasWanted from kv store to persistent kv store

* Fix typo in test name

* switch back to using kvstore, not persistent kv store

* remove gogoproto from tools

because it's not a binary

* update protobuf version to 3.6.1 in `make get_protoc`

* update libs/common/types.pb.go and rpc/grpc/types.pb.go

* fix app tests

This reverts commit d02c5d1e30.