Deprecates unused backup-connector-name attribute

in messaging subsystem, the <connector-ref> element defines an optional backup-connector-name.

This attribute was used by previous version of HornetQ but is no longer used.

We need to document the XML schema to state that this attribute is deprecated and not taken into account when configuring <connector-ref> in messaging susbsystem.
Additionnally, we can log a warning about this deprecated attribute when parsing XML configuration file.

We can also update the resource description and add a deprecated comment for the connection-factory.connector / pooled-connection-factory.connector