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

Makefile

@@ -5,7 +5,7 @@ GOTOOLS = \
 	github.com/gogo/protobuf/protoc-gen-gogo \
 	github.com/gogo/protobuf/gogoproto \
 	github.com/square/certstrap
-PACKAGES=$(shell go list ./... | grep -v '/vendor/')
+PACKAGES=$(shell go list ./...)
 
 INCLUDE = -I=. -I=${GOPATH}/src -I=${GOPATH}/src/github.com/gogo/protobuf/protobuf
 BUILD_TAGS?='tendermint'
@@ -37,7 +37,7 @@ protoc_all: protoc_libs protoc_abci protoc_grpc
 	## If you get the following error,
 	## "error while loading shared libraries: libprotobuf.so.14: cannot open shared object file: No such file or directory"
 	## See https://stackoverflow.com/a/25518702
-	protoc $(INCLUDE) $< --gogo_out=plugins=grpc:.
+	protoc $(INCLUDE) $< --gogo_out=Mgoogle/protobuf/timestamp.proto=github.com/golang/protobuf/ptypes/timestamp,plugins=grpc:.
 	@echo "--> adding nolint declarations to protobuf generated files"
 	@awk -i inplace '/^\s*package \w+/ { print "//nolint" }1' $@