[dnsdist] dnscrypt cert roller

[powerdns at email.shininet.org]

I've written a lua script to help roll over dnscrypt certs hourly.
Making it avail to anyone who might find it helpful, no claims about
how many bugs it may have.  Feel free to update and repost.

To use: call `certrot_init(privkeyfile)` on load in config, and call
`certrot_genRotate()` hourly via cron

Default valid range is from -1h to +2h. genRotate will create a new
cert, apply it to all binds, move all other certs to inactive
status, and remove them if Now is past validUntil.

< file attached and avail: https://pastebin.com/bNdygNxz >
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