Deprecate client/selector (#1767)

* client/{grpc,rpc}: depricate selector (wip)

* {client,cmd}: remove client/selector

* deprecate client/selector

* router/static: fix lookup

* config/cmd: add support for legacy static selector flag

* config/cmd: add support for legacy dns selector flag

client/client.go

@@ -22,11 +22,6 @@ type Client interface {
 	String() string
 }
 
-// Router manages request routing
-type Router interface {
-	SendRequest(context.Context, Request) (Response, error)
-}
-
 // Message is the interface for publishing asynchronously
 type Message interface {
 	Topic() string