Leaf hhash node size is growable (1-4 pages).
The rule is simple, when a hashtable is split the left and right
children size is clamped left and right weight to the nearest
hhash size. The sizes are chosen arbitrarily so the table size
is always a multiple of page size.

- misleading statement
- module name in heading
- s/forward\./example./g

Query plan modules + synthetic PTR/A/AAAA records

close #166

Gcov

This MR adds support to enable code coverage testing with:

```shell
./configure --enable-code-coverage
make
make check-code-coverage
```

The output can be found in $(PACKAGE_NAME)-$(PACKAGE_VERSION)-coverage directory.

You have to have lcov package installed for this to work.

DNSKEY in non-apex nodes

This changeset improves handling of DNSKEY records in non-apex nodes.

Prior to this change, non-apex DNSKEYs were not signed and invalid NSEC
records were generated for these nodes.