gatt: Fix not cleaning up when disconnected

gatt: Fix not cleaning up when disconnected

There is a current use after free possible on a gatt server if a client
disconnects while a WriteValue call is being processed with dbus.

This patch includes the addition of a pending disconnect callback to handle
cleanup better if a disconnect occurs during a write, an acquire write
or read operation using bt_att_register_disconnect with the cb.

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>