Change get_timestamp() to current_time()

Makes more sense language-wise to compare (current_time >
timeout_timestamp) than (get_timestamp > timeout_timestamp).
Remove classes Key and Value

In retrospect, it was kind of dumb to have custom Key and Value
objects, even if they held information about the responsible node_hash
or timestamp.  Messages now enter the system as json, have fields added
to them as needed, and are passed around as json.