Serialized SparsePoseGraphs can easily be too big for default proto deserialization

I'm working with some very large multi-trajectory SLAM problems (10s of kms, indoor), and the resulting serialized SparsePoseGraph objects are too big to deserialize, as they're much larger than the default 64MB limit. (Seriously: I'm up to about 1.3GB for the SPG alone.)

There are three solutions:

1. Modify the serialization of SPG to use the pbstream, writing the pieces out one-by-one.
2. Improve pbstream to use a google::protobuf::io::CodedInputStream directly, so we can call SetTotalBytesLimit with a bigger value.
3. "Mac, you're out of your mind, make yourself a branch and leave the rest of us out of this."

(I've currently implemented 3, via 2, in macmason/cartographer and macmason/cartographer_ros, in branches both called mass_map.)

Thoughts?

FYI @brandon-northcutt