Make net.secloud.core.BaseObject.id non optional

Change the property id of type net.secloud.core.BaseObject from
Option[ObjectId] to ObjectId for a more concistent API later on.
Enrich ObjectDatabase trait

Add two methods read and write to net.secloud.core.ObjectDatabase to
allow safe and scala idomatic usage of net.secloud.core.ObjectReader and
net.secloud.core.ObjectWriter.