Centralize Ticket management of persistent stores (#682)

* Centralize Ticket management of persistent stores

persistence package with Manager & Ticket will handle
all the details about keys, secrets, ticket into cookies,
etc. Persistent stores just need to pass Save, Load &
Clear function handles to the persistent manager now.

* Shift to persistence.Manager wrapping a persistence.Store

* Break up the Redis client builder logic

* Move error messages to Store from Manager

* Convert ticket to private for Manager use only

* Add persistence Manager & ticket tests

* Make a custom MockStore that handles time FastForwards

pkg/sessions/persistence/interfaces.go

@@ -0,0 +1,15 @@
+package persistence
+
+import (
+	"context"
+	"time"
+)
+
+// Store is used for persistent session stores (IE not Cookie)
+// Implementing this interface allows it to easily use the persistence.Manager
+// for session ticket + encryption details.
+type Store interface {
+	Save(context.Context, string, []byte, time.Duration) error
+	Load(context.Context, string) ([]byte, error)
+	Clear(context.Context, string) error
+}