datastore: protobuf backend

[beriberikix]

Add support for the highly-performant Protobuf protocol.

[rakyll]

Related to #1.

[rakyll]

I'm closing this issue for now, as it's not in the scope of M1 for this client (as it turned out to be a complicated issue). Switching to protobuf will happen as a perf improvement in the future.

[beriberikix]

Can we create/add a label so we don't lose track? This one in particular is important for other teams.

[rakyll]

I'm adding it to M2 and reopening the issue.

[rakyll]

Blocked by protobufjs/protobuf.js#179

[rakyll]

Fixed by #112.