lmdb NSEC3 record handling hygiene

[miodvallat]

Short description

This (hopefully) completes the recent NSEC3 record handling work in LMDB, to guarantee that we never leave dangling records when performing deletions or updates:

• when updating an NSEC3 record, remove the previous back record (since it would become dangling).
• when removing an NSEC3 record, always remove the other record from the pair.

Note the second item from the list above manifests itself in multiple ways in replaceRRSet:

• When removing all records regardless of their type (using QType::ANY), we need to check for an NSEC3 record and, if found, remove the matching record of the chain.
• When removing a particular record type, and not adding any record after that, we need to check whether we end up with a single NSEC3 record to that name, in which case it should be removed (with the matching record from the pair).

This should fix #11612 and give #11611 a chance to get merged soon.

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)

[coveralls]

Pull Request Test Coverage Report for Build 16052007491

Details

• 132 of 149 (88.59%) changed or added relevant lines in 3 files are covered.
• 21 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+2.6%) to 65.606%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/rfc2136handler.cc	52	57	91.23%
modules/lmdbbackend/lmdbbackend.cc	75	87	86.21%

Files with Coverage Reduction	New Missed Lines	%
pdns/rcpgenerator.cc	2	90.13%
pdns/recursordist/sortlist.cc	2	72.94%
pdns/rfc2136handler.cc	2	72.38%
modules/gpgsqlbackend/spgsql.cc	3	68.42%
pdns/recursordist/syncres.cc	3	80.34%
pdns/remote_logger.cc	3	56.7%
modules/godbcbackend/sodbc.cc	6	76.03%

Totals
Change from base Build 16051915340:	2.6%
Covered Lines:	126980
Relevant Lines:	164867

---

💛 - Coveralls