vendor: Update github.com/gogo/protobuf

Also tweaks the proto definitions:

 - [packed=false] on the block_indexes field to retain compat with
   v0.14.16 and earlier.

 - Uses the vendored protobuf package in include paths.

And, "build.go setup" will install the vendored protoc-gen-gogofast.
This should ensure that a proto rebuild isn't so dependent on whatever
version of the compiler and package the developer has installed...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3864

vendor/github.com/gogo/protobuf/test/theproto3/footer.proto

@@ -0,0 +1,25 @@
+
+message MessageWithMap {
+  map<int32, string> name_mapping = 1;
+  map<sint64, FloatingPoint> msg_mapping = 2;
+  map<bool, bytes> byte_mapping = 3;
+}
+
+message FloatingPoint {
+  double f = 1;
+}
+
+message Uint128Pair {
+  bytes left = 1 [(gogoproto.nullable) = false, (gogoproto.customtype) = "github.com/gogo/protobuf/test/custom.Uint128"];
+  bytes right = 2 [(gogoproto.customtype) = "github.com/gogo/protobuf/test/custom.Uint128"];
+}
+
+message ContainsNestedMap {
+	message NestedMap {
+		map<string, double> NestedMapField = 1;
+	}
+}
+
+message NotPacked {
+	repeated uint64 key = 5 [packed=false];
+}