proto3 timestamp (#2064)

This PR changes ABCI time format from int64 (Unix seconds) to WKT (WellKnownType) google.protobuf.Timestamp.

Refs #1857

Reasons:

better precision
standard DT for proto

* update Gopkg.lock
* [makefile] remove extra grep
    - go list excludes vendor by default now
* proto3 timestamp
* [docs/abci-spec] note about serialisation format
* make time non-nullable

abci/tests/test_app/app.go

@@ -26,7 +26,7 @@ func startClient(abciType string) abcicli.Client {
 }
 
 func setOption(client abcicli.Client, key, value string) {
-	_, err := client.SetOptionSync(types.RequestSetOption{key, value})
+	_, err := client.SetOptionSync(types.RequestSetOption{Key: key, Value: value})
 	if err != nil {
 		panicf("setting %v=%v: \nerr: %v", key, value, err)
 	}