[WIP] Store Sync (#1365)

* Initial cache implementation

* Write queue implementation

* Accidentally started writing the storage sync service

sync/store/cache_test.go

@@ -0,0 +1,26 @@
+package store
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/micro/go-micro/v2/store"
+	"github.com/micro/go-micro/v2/store/memory"
+)
+
+func TestCacheTicker(t *testing.T) {
+	l0 := memory.NewStore()
+	l0.Init()
+	l1 := memory.NewStore()
+	l1.Init()
+	l2 := memory.NewStore()
+	l2.Init()
+	c := NewCache(Stores(l0, l1, l2), SyncInterval(1*time.Second), SyncMultiplier(2))
+
+	if err := c.Init(store.WithContext(context.Background())); err != nil {
+		t.Fatal(err)
+	}
+
+	time.Sleep(30 * time.Second)
+}