Mosquitto stats align

The current $SYS/ stats have been aligned to the Mosquitto ones.

In this branch, it publishes:
```
$SYS/e22b868a/version mosca 0.17.2
$SYS/e22b868a/uptime 8 minutes
$SYS/e22b868a/connections 2
$SYS/e22b868a/publish/received 95
$SYS/e22b868a/load/connections/15m 0
$SYS/e22b868a/load/publish/received/15m 0
$SYS/e22b868a/load/connections/5m 6
$SYS/e22b868a/load/publish/received/5m 56
$SYS/e22b868a/load/connections/1m 0
$SYS/e22b868a/load/publish/received/1m 13
$SYS/e22b868a/memory/rss 48640000
$SYS/e22b868a/memory/heapUsed 21333584
$SYS/e22b868a/memory/heapTotal 34235136
```

@andypiper and @ralight what do you think?

@ralight in the 'connections' load, you track the total number of connections at that given instant, or the new connections in that period? I'm tracking the latter, but of course I might get some zeros while there are connected clients.