Add remaining client/worker handlers.

Adds all remaining handle methods needed to handle responses that
clients and workers can expect.

Adds an echo method to connection objects (for ping tests).

Adds all packet types to the handle method in BaseClientServer,
and add default implementations for each of them that logs an error.
Subclasses (eg Client, Worker) will override them as appropriate.

Add a "broadcast lock" so that Clients and Workers can maintain
exclusivity around operations that apply to all connections.

Change-Id: Ia31f0941687ccfee711c3818e8e0c21ccbd1b313