mailq: Warn on age, not only on count

[xeiss]

Describe the solution you'd like

Additional warn/crit alert possibility for the age of the oldest mail in queue. Default 1 Hour warn, no crit per default.
Warn count default could then be higher, maybe 10.
Perfdata could be still the count, but also maybe the age of the oldest mail in seconds.

Additional context

For my mailq monitoring i don't really want to get an alert on the count. Because 100 mails in queue are fine (on some systems), when they will get sent within some minutes. But already 1 mail, which is stuck for more then one hour is for me more interessting in monitoring.

[markuslf]

Thank you for your suggestion, sounds reasonable.