EXAMPLES: mailqueue for lua mailers script

EXAMPLES: mailqueue for lua mailers script

Lua mailers scripts now leverages Queue class to implement a mailqueue.

Thanks to the mailqueue, emails are still sent asynchronously, but with
respect to their generation order (better ordering consistency).

That is, previous script limitation (see below) is no longer true:
"
 Current known script limitation: if multiple events are generated
 simultaneously it is possible that emails could be received out of
 order since emails are sent asynchronously using smtp_send_email()
 and there is no sending queue. Relying on the email "date" should
 help to know which email was generated first..
"

For a given server, email-alerts will be sent in the same order as the
events were generated. However this does not apply to events between 2
distinct servers, since each server is tracked independently within
the lua script. (1 subscription per server, to make sure only relevant
servers x events are being tracked and prevent useless wakeups)