Rename and refactor the types passed over the API to be based on an
entity that's either living or a tombstone. A living entity has a
deleted property that's either undefined or false, while a tombstone
has a deleted property set to true. All entities have a numeric id
and an updatedAt timestamp.
To sync entities, an array of replacements are passed around. Living
entities are replaced with tombstones when they're deleted. And
tombstones are replaced with living entities when restored.
Start the work of clearly distingushing client side types, server side
types and types shared over the API by renaming "AccountSession" and
"Session" names used on the server to "ServerSession".
