This is logically part of the next commit. But then, the git diff would
show the old file disappear and the new file appear with new content,
which would be hard to review, so it is split. This commit should
contain no changes in the content (hopefully), so can be skipped during
review.

When changing the names, this kind of got forgotten.

Including the expected usage scenarios.

Also, the interface was slightly refined - renaming of things, adding
support for the RRSig iteration.

The oposite of the isc::datasrc::memory::RDataEncoder.

Only the methods are drafted, no documentation yet and no member
variables listed.

The previous version was slightly misleading, the reader could
understand it the queries would be just slow as answered from the DB,
instead of not answered at all.

since the b10-showtech had not been released yet, I just replaced its name in the existing changelog entry

also changed the category from bug to build (it was definitely not a bug).
okayed on jabber.

Which is still used in some places.

Describe the data sources in the guide.