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/CONTRIBUTORS

@@ -0,0 +1,16 @@
+Anton Povarov <anton.povarov@gmail.com>
+Clayton Coleman <ccoleman@redhat.com>
+Denis Smirnov <denis.smirnov.91@gmail.com>
+DongYun Kang <ceram1000@gmail.com>
+Dwayne Schultz <dschultz@pivotal.io>
+Georg Apitz <gapitz@pivotal.io>
+Gustav Paul <gustav.paul@gmail.com>
+Johan Brandhorst <johan.brandhorst@gmail.com>
+John Tuley <john@tuley.org>
+Laurent <laurent@adyoulike.com>
+Patrick Lee <patrick@dropbox.com>
+Stephen J Day <stephen.day@docker.com>
+Tamir Duberstein <tamird@gmail.com>
+Todd Eisenberger <teisenberger@dropbox.com>
+Tormod Erevik Lea <tormodlea@gmail.com>
+Walter Schulze <awalterschulze@gmail.com>