* Use nightly protoc gRPC plugin for Node
Newer versions of the Node gRPC plugin accept the 'minimum_node_version'
flag, which we can use to instruct protoc to not support Node versions
earlier than Node 6. This allows the compiler to use 'Buffer.from'
instead of the deprecated 'Buffer' constructor, which fixes a
deprecation warning on Node 10.
* Protobuf changes
I tried to re-generate the Protobuf/gRPC files, but generate.sh seems to
have assumed I manually built the Dockerfile ahead-of-time (unless I'm
missing something -- very possible). Unfortunately, when I went to do
that, I ended up with a handful of errors, most of them due to
differences in versions. (We probably want to think about pinning them.)
To remedy both issues, I've fixed up the Dockerfile so that it works for
me at least, and added a build step to the front of generate.sh.
