selector: new selector interface with random & roundrobin implementation (#1761)

* selector: implement new selector interface plus random & roundrobin implementations

* selector/roundrobin: remove unused consts

* router: add close method to interface

* selector/roundrobin: fix concurrent map iteration and map write

* selector: replace variadic argument on Select

selector/selector.go

@@ -0,0 +1,42 @@
+package selector
+
+import (
+	"errors"
+
+	"github.com/micro/go-micro/v2/router"
+)
+
+var (
+	// DefaultSelector is the default selector
+	DefaultSelector = NewSelector()
+
+	// ErrNoneAvailable is returned by select when no routes were provided to select from
+	ErrNoneAvailable = errors.New("none available")
+)
+
+// Selector selects a route from a pool
+type Selector interface {
+	// Init a selector with options
+	Init(...Option) error
+	// Options the selector is using
+	Options() Options
+	// Select a route from the pool using the strategy
+	Select([]*router.Route) (*router.Route, error)
+	// Record the error returned from a route to inform future selection
+	Record(*router.Route, error) error
+	// Close the selector
+	Close() error
+	// String returns the name of the selector
+	String() string
+}
+
+// Options used to configure a selector
+type Options struct{}
+
+// Option updates the options
+type Option func(*Options)
+
+// NewSelector creates new selector and returns it
+func NewSelector(opts ...Option) Selector {
+	return newSelector(opts...)
+}