Add content and snapshot store references

Update database object to hold reference to the
data stores.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

metadata/db.go

@@ -1,14 +1,23 @@
 package metadata
 
-import "github.com/boltdb/bolt"
+import (
+	"sync"
+
+	"github.com/boltdb/bolt"
+)
 
 type DB struct {
 	db *bolt.DB
+
+	storeL sync.Mutex
+	ss     map[string]*snapshotter
+	cs     *contentStore
 }
 
 func NewDB(db *bolt.DB) *DB {
 	return &DB{
 		db: db,
+		ss: map[string]*snapshotter{},
 	}
 }