rrset writing: remove KNOT_RRSET_WIRE_CANONICAL, remove flags

All data in server are kept in canonical format after recent
changes.The flag is no longer needed.

Existing code didn't recognize case sensitive RR types (LP),
which caused creation of invalid signatures.
DNSSEC tests: new test for case sensitivity of domain names
DNSSEC tests: remove some cases from no_resign covered by case_sensitivity
Merge branch 'fix-lp-signing' into 'master'

fix LP signing

See merge request !293