Polluting DEBUG logs with $SYS published messages

[behrad]

This line is really log heavy since is logging all $SYS messages: https://github.com/mcollina/mosca/blob/master/lib/server.js#L357

In my fork I was changing $SYS topic publishes to .trace, I'd forgotten to create a PR, but I'm going to create one if you don't mind @mcollina

[behrad]

Can be changed to this

if( packet.topic.indexOf( '$SYS' ) >= 0 ) {
  logger.trace({ packet: packet }, "published packet");
} else {
  logger.debug({ packet: packet }, "published packet");
}