[Feat]: Add Data point for postfix collector "Average delay"

Problem

Netdata's Postfix collector only offers information about the queue size by data storage and count.

Description

I would like to have a 3rd data point: "average delay". I want to know what's the average age of the message is sitting in the queue and have that plotted over time.

Importance

really want

Value proposition

1. No reason to use 3rd party outdated (Perl based) stats tools to get this information
2. Adds more insight into the health of your mail server
3. Keeps data in one place
4. Allows you to setup an alarm if average message age (aka delay) is too high
5. Improve user experience by allowing system administrators to proactively monitor for mail delays.

Proposed implementation

It's possible to calculate a delay data point using the information available from postque. There is a property called arrival_time that contains the Unix timestamp of when the message was first seen.

This information can be used to calculate an average delivery delay in a given period for all mail in the queue at the time of polling.