OGM-605 Fix composite many to many for collection per association and global
OGM-605 Move ManyToOne composite tests to ManyToOne test

That way it is shared by all dialect specialized subclasses
like MongoDB
OGM-605 Rename MongoDB format test for association and composite keys
OGM-605 Flatten List and composite id columns in MongoDB representation

{ "id": { "id1":"foo", "id2":"bar" }, "rank":0 }
=>
{ "id1":"foo", "id2":"bar", "rank":0 }