Adding object table (#1)

* code for maintaining the object table

* Makefile fix

* Clone git submodules.

* directory -> object_table

* Fix Makefile and remove unnecessary files.

* Fix formatting.

* make code more generic

state/object_table.h

@@ -0,0 +1,15 @@
+#include "common.h"
+#include "db.h"
+
+typedef void (*lookup_callback)(void *);
+
+/* Register a new object with the directory. */
+void object_table_add(db_conn *db, unique_id object_id);
+
+/* Remove object from the directory */
+void object_table_remove(db_conn *db, unique_id object_id);
+
+/* Look up entry from the directory */
+void object_table_lookup(db_conn *db,
+                         unique_id object_id,
+                         lookup_callback callback);