add metastore, switching sequence to use it

metastore is for storing metadata. This will be used later when moving
to distributed master mode.

go/metastore/backing_test.go

@@ -0,0 +1,30 @@
+package metastore
+
+import (
+	"testing"
+)
+
+func TestMemoryBacking(t *testing.T) {
+	ms := &MetaStore{NewMetaStoreMemoryBacking()}
+	verifySetGet(t, ms)
+}
+
+func TestFileBacking(t *testing.T) {
+	ms := &MetaStore{NewMetaStoreFileBacking()}
+	verifySetGet(t, ms)
+}
+
+func verifySetGet(t *testing.T, ms *MetaStore) {
+	data := uint64(234234)
+	ms.SetUint64(data, "/tmp", "sequence")
+	if !ms.Has("/tmp", "sequence") {
+		t.Errorf("Failed to set data")
+	}
+	if val, err := ms.GetUint64("/tmp", "sequence"); err == nil {
+		if val != data {
+			t.Errorf("Set %d, but read back %d", data, val)
+		}
+	} else {
+		t.Errorf("Failed to get back data:%s", err)
+	}
+}