TcReader<T,M> maintains its local T object. The reader will receive and
apply incoming M messages to its T whenever explicitly invoked by
update, update_limited or implicity by get_mut_fresh.
A set of connected TcWriter<M>s have any number (initially 0) of reading
TcWriter<T,M> objects for any types T (typically the same type T).
The data within some reader r can be then accessed by dereferencing *r.