Fix directory entry creation when using schema with a prefix

Directory.createEntry(DocumentModel entry) breaks if the directory schema has a non null prefix

All directory implementations of the createEntry(DocumentModel entry) method (SQL, LDAP and Multi do not handle correctly) schemas with a prefix: the fieldnames will carry the prefix while the internal field map do not expect it.

Also this method is copy and pasted and should be factorized in the BaseSession abstract base class instead.