Rewrite ejabberd system monitor

Rewrite ejabberd system monitor

Previous version was inefficient: it had a lot of false positives
along with a lot of false negatives, making its usage pointless.
The new verion is based on memsup(3erl) application: the OOM watchdog is
only started when total OS memory consumption is more than 80%.
A watchdog periodically inspects all running processes and collects
statistics about overloaded ones (those queueing a lot of messages).
If the OOM killer is enabled (`oom_killer: true`), all overloaded
processes would be killed. By default, OOM killer is enabled.
When memory consumption is back to normal, the OOM watchdog is stopped.