Generate protobuf files during compilation and fix the generation rules.

The change achieves the following:
1. Adds generation of Go and Python protobuf files during build by adding
   dependency of 'build' rule on 'proto' rule. This will avoid surprises of
   'make build' giving error when proto file is changed.
2. Modifies the generation of protobuf files to be more in line of how Makefiles
   should be written. This has an additional benefit that protobuf files won't
   be regenerated if proto files did not change.
3. Modifies the generation rules to work both locally on a workstation and
   inside the docker image. Docker image doesn't have /vt/dist/grpc installed,
   so the existing rule didn't work.

Note that as a result of this a new directory go/vt/.proto.tmp will exist after
building. I believe that should be fine and won't interfere with anything.

.gitignore

@@ -62,3 +62,6 @@ _test/
 
 #pom generated file
 /java/jdbc/dependency-reduced-pom.xml
+
+# generated protobuf files
+/go/vt/.proto.tmp

Makefile

@@ -8,7 +8,7 @@ MAKEFLAGS = -s
 # Since we are not using this Makefile for compilation, limiting parallelism will not increase build time.
 .NOTPARALLEL:
 
-.PHONY: all build test clean unit_test unit_test_cover unit_test_race integration_test proto site_test site_integration_test docker_bootstrap docker_test docker_unit_test java_test php_test reshard_tests
+.PHONY: all build test clean unit_test unit_test_cover unit_test_race integration_test proto proto_banner site_test site_integration_test docker_bootstrap docker_test docker_unit_test java_test php_test reshard_tests
 
 all: build test
 
@@ -26,7 +26,7 @@ ifdef VT_MYSQL_ROOT
   endif
 endif
 
-build:
+build: proto
 ifndef NOBANNER
 	echo $$(date): Building source tree
 endif
@@ -43,6 +43,7 @@ site_test: unit_test site_integration_test
 clean:
 	go clean -i ./go/...
 	rm -rf third_party/acolyte
+	rm -rf go/vt/.proto.tmp
 
 # This will remove object files for all Go projects in the same GOPATH.
 # This is necessary, for example, to make sure dependencies are rebuilt
@@ -94,15 +95,45 @@ install_protoc-gen-go:
 	cp -au vendor/github.com/golang/protobuf $${GOPATH}/src/github.com/golang/
 	go install github.com/golang/protobuf/protoc-gen-go
 
-# This rule rebuilds all the go files from the proto definitions for gRPC.
-# 1. list all proto files.
-# 2. remove 'proto/' prefix and '.proto' suffix.
-# 3. (go) run protoc for each proto and put in go/vt/proto/${proto_file_name}/
-# 4. (python) run protoc for each proto and put in py/vtproto/
-proto: install_protoc-gen-go
-	find proto -maxdepth 1 -name '*.proto' -print | sed 's/^proto\///' | sed 's/\.proto//' | xargs -I{} $$VTROOT/dist/grpc/usr/local/bin/protoc -Iproto proto/{}.proto --go_out=plugins=grpc:go/vt/proto/{}
-	find go/vt/proto -name "*.pb.go" | xargs sed --in-place -r -e 's,import ([a-z0-9_]+) ".",import \1 "github.com/youtube/vitess/go/vt/proto/\1",g'
-	find proto -maxdepth 1 -name '*.proto' -print | sed 's/^proto\///' | sed 's/\.proto//' | xargs -I{} $$VTROOT/dist/grpc/usr/local/bin/protoc -Iproto proto/{}.proto --python_out=py/vtproto --grpc_out=py/vtproto --plugin=protoc-gen-grpc=$$VTROOT/dist/grpc/usr/local/bin/grpc_python_plugin
+PROTOC_DIR := $(VTROOT)/dist/grpc/usr/local/bin
+PROTOC_EXISTS := $(shell type -p $(PROTOC_DIR)/protoc)
+ifeq (,$(PROTOC_EXISTS))
+  PROTOC_BINARY := $(shell which protoc)
+  ifeq (,$(PROTOC_BINARY))
+    $(error "Cannot find protoc binary. Did you execute 'source dev.env'?")
+  endif
+  PROTOC_DIR := $(dir $(PROTOC_BINARY))
+endif
+
+PROTO_SRCS = $(wildcard proto/*.proto)
+PROTO_SRC_NAMES = $(basename $(notdir $(PROTO_SRCS)))
+PROTO_PY_OUTS = $(foreach name, $(PROTO_SRC_NAMES), py/vtproto/$(name)_pb2.py)
+PROTO_GO_OUTS = $(foreach name, $(PROTO_SRC_NAMES), go/vt/proto/$(name)/$(name).pb.go)
+PROTO_GO_TEMPS = $(foreach name, $(PROTO_SRC_NAMES), go/vt/.proto.tmp/$(name).pb.go)
+
+# This rule rebuilds all the go and python files from the proto definitions for gRPC.
+proto: proto_banner $(PROTO_GO_OUTS) $(PROTO_PY_OUTS)
+
+proto_banner:
+ifndef NOBANNER
+	echo $$(date): Compiling proto definitions
+endif
+
+$(PROTO_PY_OUTS): py/vtproto/%_pb2.py: proto/%.proto
+	$(PROTOC_DIR)/protoc -Iproto $< --python_out=py/vtproto --grpc_out=py/vtproto --plugin=protoc-gen-grpc=$(PROTOC_DIR)/grpc_python_plugin
+
+$(PROTO_GO_OUTS): $(PROTO_GO_TEMPS)
+	for name in $(PROTO_SRC_NAMES); do \
+	  mkdir -p go/vt/proto/$${name}; \
+	  cp -a go/vt/.proto.tmp/$${name}.pb.go go/vt/proto/$${name}/$${name}.pb.go; \
+	done
+
+$(PROTO_GO_TEMPS): install_protoc-gen-go
+
+$(PROTO_GO_TEMPS): go/vt/.proto.tmp/%.pb.go: proto/%.proto
+	mkdir -p go/vt/.proto.tmp
+	$(PROTOC_DIR)/protoc -Iproto $< --go_out=plugins=grpc:go/vt/.proto.tmp
+	sed --in-place -r -e 's,import ([a-z0-9_]+) ".",import \1 "github.com/youtube/vitess/go/vt/proto/\1",g' $@
 
 # Generate the PHP proto files in a Docker container, and copy them back.
 php_proto: