feat(dns): support fixed domain ttl

[mzz2017]

Background

Some DDNS domains want to disable DNS cache or want less TTL.

Checklist

• The Pull Request has been fully tested
• There is a user-facing docs PR against https://github.com/daeuniverse/dae

Full changelog

1. Remove min 120 ttl limit.
2. Allow to overwrite TTL of specific domains.
3. Remove protection that new cache deadline must be greater than old one.

See (in dns section)

dae/example.dae

Lines 119 to 125 in a26f054

	# Give a fixed ttl for domains. Zero means that dae will request to upstream every time and not cache DNS results
	# for these domains.
	#fixed_domain_ttl {
	# ddns.example.org: 10
	# test.example.org: 3600
	#}

Issue reference

Fix #96