Storage has now it's own package.

Remove global `sessions` map and use configured storage.
Add a `types` package so both `pwd` and `storage` can access without
circular dependencies.
Now the session is prepared when requested and not on load.

storage/storage.go

@@ -0,0 +1,23 @@
+package storage
+
+import "github.com/play-with-docker/play-with-docker/pwd/types"
+
+const notFound = "NotFound"
+
+func NotFound(e error) bool {
+	return e.Error() == notFound
+}
+
+type StorageApi interface {
+	SessionGet(sessionId string) (*types.Session, error)
+	SessionPut(*types.Session) error
+	SessionCount() (int, error)
+	SessionDelete(sessionId string) error
+
+	InstanceFindByAlias(sessionPrefix, alias string) (*types.Instance, error)
+	// Should have the session id too, soon
+	InstanceFindByIP(ip string) (*types.Instance, error)
+	InstanceCount() (int, error)
+
+	ClientCount() (int, error)
+}