Gate LinkingMetadataMBean on a config directive

[breedx-nr]

This should be built after #61.

Right now, we always try and register the LinkingMetadataMBean with the platform MBean server. In some configurations, this can be problematic and prevent the agent from starting. We should change this behavior so that the LInkingMetadataMBean is only registered when a config item is present.

This config item should be part of the jmx subconfig, perhaps something like jmx.registerLinkingMetadataMBean or similar.