[MemorySpool] Cloning messages to the queue

This PR is a merge of #278 and #518. With help from @xdecock we got the cloning to work. 

I need some advice about the failing test. I suspect this is a false positive. Am I correct when saying that properties of the message does not need to be cloned? If not, the test is valid and we (or I ?) need to do some more changes to this PR. 
A clone gets a new id. This invalidates any links to `cid`s that has been put in the body of the email.

See bug issue #534