dnsdist: Add the ability to dump a summary of the cache content

[rgacogne]

Short description

The new feature writes a summary of the packet cache content to a file, dumping for each entry:

• qname
• remaining TTL
• qtype
• computed hash
• response's length
• TCP or UDP
• the timestamp of when the entry was added to the cache.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[phonedph1]

This is much better than what I had come up with!

[pieterlexis]

Looks good. Perhaps the TTL and QType should be swapped?

Also, I would like a 'header' on the first line like this:

; qname TTL type

[rgacogne]

Perhaps the TTL and QType should be swapped?

I had the same feeling but decided to keep the same order than the one when dumping the recursor's packet cache. But the more I think about it, the less relevant it is so I'm OK with swapping it.

Also, I would like a 'header' on the first line like this:

I'm not sure it's really needed, because that's almost what we already have:

; dnsdist's packet cache dump follows
;
powerdns.com. 3572 A ; key 2000355501, length 57, tcp 0, added 1529912626
www.powerdns.com. 3590 A ; key 4247463626, length 61, tcp 0, added 1529912644
nx.powerdns.com. 3595 A ; key 1069543761, length 109, tcp 0, added 1529912649