Base class for the server implementation of the service Controller2.
The Controller service provides the API for orchestrating a collection of
debugger agents to perform debugging tasks. These agents are each attached
to a process of an application which may include one or more replicas.
The debugger agents register with the Controller to identify the application
being debugged, the Debuggee. All agents that register with the same data,
represent the same Debuggee, and are assigned the same `debuggee_id`.
The debugger agents call the Controller to retrieve the list of active
Breakpoints. Agents with the same `debuggee_id` get the same breakpoints
list. An agent that can fulfill the breakpoint request updates the
Controller with the breakpoint result. The controller selects the first
result received and discards the rest of the results.
Agents that poll again for active breakpoints will no longer have
the completed breakpoint in the list and should remove that breakpoint from
their attached process.
The Controller service does not provide a way to retrieve the results of
a completed breakpoint. This functionality is available using the Debugger
service.