Move the logic that converts the EntityClass of a map to a string and
then back into the class to the payload plugin in order to avoid a
circular dependency where the ClientMap needs to know the entity classes
and the entity classes needs to know the ClientMap.
The only place that doesn't know the type of the entities stored in the
client map is the payload reviver, so it makes sense to keep this logic
contained to the payload plugin.
Create a simple Map like class for storing and keeping track of client
entities that are synced from the server and have local editable state.
This will form the basis for storing entities on the client and should
replace the immutable concept used be the ClientSchedule class.
