Add a helper function to de-serialize Protocol Buffer VarInt32.

The header in each response is prefixed by a varint32 so we need a
method to read those.  This implementation is about 30% faster than
that in the Google protobuf library, and was proposed upstream:
https://code.google.com/p/protobuf/issues/detail?id=534
WIP: Convert META lookups to protobuf.