Starting to add the acquaintance concept

Checkpoint: doesn't do anything yet, but it does compile.
Pull the CollaboratorCache out to a separate Actor

This permits better separation of concerns, and helps avoid the
UserSession getting too bogged-down.
CONFIG CHANGE: Added the front end to prompt for collaborators

Also, dealt with several important edge cases.

Added Config.getDuration, so we have an official way to do this.

Need to add config entry for the timeout on the collaborators cache.
Made inviteMembers async

The asynchrony is stacking up in there, so decided to account for it
properly before it overwhelms things.

Began to introduce collaborators to the stack, but we're not actually
using them yet.
Invite Collaborators works!

Yay!

Added email address to FullIdentity (it's decently appropriate), and
introduced a way for components in the identity namespace to fetch
FullIdentity from the IdentityCache.
Fix the test stubs
Release 0.10.4