cleanup api server struct creation

[michel-laterman]

The api.Server's creation signature is:

NewServer(addr string, cfg *config.Server, ct *CheckinT, et *EnrollerT, at *ArtifactT, ack *AckT, st *StatusT, sm policy.SelfMonitor, bi build.Info, ut *UploadT, ft *FileDeliveryT, pt *PGPRetrieverT, bulker bulk.Bulk, tracer *apm.Tracer) *server

This leads to test cases where we pass a lot of nil args:

srv := NewServer(addr, cfg, nil, nil, nil, nil, nil, nil, fbuild.Info{}, nil, nil, nil, nil, nil)

We should change the signature of NewServer to take a struct that has the pointers to all the route handlers, i.e.,

NewServer(addr string, cfg *config.Server, bi build.Info, handlers *Handlers)

or use optional args, i.e.,

NewServer(addr string, cfg *config.Server, bi build.Info, ...options Option)

We can also try to change where we apply the handler rate-limiters to get rid of the pathToOperation function and clean up how the endpoints are wrapped.