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