add ABCI interface for state sync snapshots (#4704)

Adds the ABCI interface for [state sync](https://github.com/tendermint/tendermint/issues/828) as outlined in [ADR-053](https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-053-state-sync-prototype.md), and bumps ABCIVersion to `0.17.0`.

The interface adds a new ABCI connection which Tendermint can use to query and load snapshots from the app (for serving snapshots to other nodes), and to offer and apply snapshots to the app (for state syncing a local node from peers).

Split out from the original PR in #4645, state sync reactor will be submitted as a separate PR. The interface is implemented by the Cosmos SDK in https://github.com/cosmos/cosmos-sdk/pull/5803.

abci/example/kvstore/persistent_kvstore.go

@@ -144,6 +144,26 @@ func (app *PersistentKVStoreApplication) EndBlock(req types.RequestEndBlock) typ
 	return types.ResponseEndBlock{ValidatorUpdates: app.ValUpdates}
 }
 
+func (app *PersistentKVStoreApplication) ListSnapshots(
+	req types.RequestListSnapshots) types.ResponseListSnapshots {
+	return types.ResponseListSnapshots{}
+}
+
+func (app *PersistentKVStoreApplication) LoadSnapshotChunk(
+	req types.RequestLoadSnapshotChunk) types.ResponseLoadSnapshotChunk {
+	return types.ResponseLoadSnapshotChunk{}
+}
+
+func (app *PersistentKVStoreApplication) OfferSnapshot(
+	req types.RequestOfferSnapshot) types.ResponseOfferSnapshot {
+	return types.ResponseOfferSnapshot{Result: types.ResponseOfferSnapshot_abort}
+}
+
+func (app *PersistentKVStoreApplication) ApplySnapshotChunk(
+	req types.RequestApplySnapshotChunk) types.ResponseApplySnapshotChunk {
+	return types.ResponseApplySnapshotChunk{Result: types.ResponseApplySnapshotChunk_abort}
+}
+
 //---------------------------------------------
 // update validators