In particular, fix the description of ServiceChangeTracker.Update's
return value, and point out that it's different from
EndpointsChangeTracker.EndpointSliceUpdate's (though fortunately, in a
way that doesn't matter for any existing code).
Put the ServiceChangeTracker and EndpointsChangeTracker definitions at
the top of the files, and put the ServicePortMap and EndpointsMap
definitions before their methods.
(No code changes.)
Move the ServicePort/BaseServicePortInfo types to serviceport.go.
Move the Endpoint/BaseEndpointInfo types to endpoint.go.
To avoid confusion with the new filenames, rename service.go to
servicechangetracker.go and endpoints.go to endpointschangetracker.go.
(No code changes; this just moves some code from types.go and
services.go to serviceport.go, and some code from types.go and
endpoints.go to endpoint.go.)
