Feature/offline multi session

[nathanhhughes]

There's some stuff that we need to clean up, but figured I'd get the multi-session changes into a PR to see if there was any design feedback. Things to think about:

• Better (more efficient) storage
• A good set of default configs somewhere in the package (I got confused by the example configs)
• Better support for loading session(s) and restoring / recomputing cached local features and/or querying against sessions on disk
• ROS multi-session support
• Thinking through more modular building blocks for the key pieces of the server (global descriptors, local features, verification and pose recovery)

[nathanhhughes]

@yunzc Feel free to ignore, I know you have a lot on your plate right now