improved direct_plugin_exports
improved direct_plugin_exports

- the queue pid is 'now' stored in the process dictionary of the process
calling the plugin register function. as a consequence the subscribe
function MUST be used from the same process.

erlio/vernemq,mururu/vmq_server,erlio/vernemq,erlio/vernemq,dergraf/vmq_server,dergraf/vmq_server,sunnyjain1/vmq_server,dejavu1988/vmq_server,linearregression/vmq_server,erlio/vmq_server,larshesel/vmq_server,erlio/vernemq,larshesel/vmq_server,erlio/vmq_server