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/file_backing.go

@@ -0,0 +1,34 @@
+package metastore
+
+import (
+	"io/ioutil"
+	"os"
+	"path"
+)
+
+// store data on disk, enough for most cases
+
+type MetaStoreFileBacking struct {
+}
+
+func NewMetaStoreFileBacking() MetaStoreFileBacking {
+	mms := MetaStoreFileBacking{}
+	return mms
+}
+
+func (mms MetaStoreFileBacking) Set(val []byte, elem ...string) error {
+	return ioutil.WriteFile(path.Join(elem...), val, 0644)
+}
+
+func (mms MetaStoreFileBacking) Get(elem ...string) (val []byte, err error) {
+	return ioutil.ReadFile(path.Join(elem...))
+}
+
+func (mms MetaStoreFileBacking) Has(elem ...string) (ok bool) {
+	seqFile, se := os.OpenFile(path.Join(elem...), os.O_RDONLY, 0644)
+	if se != nil {
+		return false
+	}
+	defer seqFile.Close()
+	return true
+}