No Default Nsec3 Tree

Two parts:

* Growable hash node size in trie
* NSEC3 tree created only if needed (NULL is treated as an empty tree)
  * I'm not sure if I did the zone-diff stuff right? :suspect:

http://www.ucw.cz/gitweb/?p=libucw.git;a=blob;f=ucw/bigalloc.c;hb=HEAD

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.

Don't merge RRs in the incoming packets

- 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.