Modified unique constraint for identifier

The getNodeByIdentifier method in Jackalope\Transport\DoctrineDBAL\Client fetches a node using the identifier and the workspace_name (which it should). However, the unique constraint only contains the identifier, which results in false positives when checking if a node doesn't exist yet. This updated constraint fixes that issue.
i think you are right, yes. a node can be cloned between workspaces, and in that case it has the same identifier in several workspaces (though semantically it is still linked and considered the "same" node).

will just wait for travis and if its green i will merge. thanks for spotting.