Add pkg/discovery for nodes discovery

Absorb Swarm's discovery package in order to provide a common node
discovery mechanism to be used by both Swarm and networking code.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

discovery/discovery.go

@@ -0,0 +1,35 @@
+package discovery
+
+import (
+	"errors"
+	"time"
+)
+
+var (
+	// ErrNotSupported is returned when a discovery service is not supported.
+	ErrNotSupported = errors.New("discovery service not supported")
+
+	// ErrNotImplemented is returned when discovery feature is not implemented
+	// by discovery backend.
+	ErrNotImplemented = errors.New("not implemented in this discovery service")
+)
+
+// Watcher provides watching over a cluster for nodes joining and leaving.
+type Watcher interface {
+	// Watch the discovery for entry changes.
+	// Returns a channel that will receive changes or an error.
+	// Providing a non-nil stopCh can be used to stop watching.
+	Watch(stopCh <-chan struct{}) (<-chan Entries, <-chan error)
+}
+
+// Backend is implemented by discovery backends which manage cluster entries.
+type Backend interface {
+	// Watcher must be provided by every backend.
+	Watcher
+
+	// Initialize the discovery with URIs, a heartbeat and a ttl.
+	Initialize(string, time.Duration, time.Duration) error
+
+	// Register to the discovery.
+	Register(string) error
+}