Add mutable extra proctype attributes

This adds mutable proctype attributes that are stored as base64-encoded
protobufs in Doozerd. The first use will be for storing per-proctype
memory limits.

I tried to keep naming and other conventions close to Matt's as-yet
uncommitted pull request to extend revision metadata:

https://github.com/soundcloud/visor/pull/86/files

For now the situation is a bit ugly, as we have three ways of encoding
information about entities in Doozerd:

- traditional file / value pairs in Doozerd
- JSON-encoded "attrs" as used for application metadata
- base64-encoded protobufs for mutable proctype attributes
  (and for revision metadata, when Matt's PR goes in)

In the long run, we should at least consolidate the JSON and
protobuf/base64 encodings.