Create a new mutable ClientEntity type and implement ClientUser on top
of it. The mutable concept is intended to replace the immutable concept
used by the ClientSchedule entities as updating immutable types in a
deep interconnected structure is a lot of hassle for little benefit.
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.
