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/db.h

@@ -0,0 +1,29 @@
+#ifndef DB_H
+#define DB_H
+
+#include "event_loop.h"
+
+typedef struct db_conn_impl db_conn;
+
+/* Connect to the global system store at address and port. The last
+ * parameter is an output parameter and we assume the memory is
+ * allocated by the caller. */
+void db_connect(const char *db_address,
+                int db_port,
+                const char *client_type,
+                const char *client_addr,
+                int client_port,
+                db_conn *db);
+
+/* Attach global system store onnection to event loop. Returns the index of the
+ * connection in the loop. */
+int64_t db_attach(db_conn *db, event_loop *loop, int connection_type);
+
+/* This function will be called by the user if there is a new event in the
+ * event loop associated with the global system store connection. */
+void db_event(db_conn *db);
+
+/* Disconnect from the global system store. */
+void db_disconnect(db_conn *db);
+
+#endif