[shmemfix2] introduce ZoneDataLoader, class version loadZoneData().

this is a minor refactoring.  the function versions were now removed,
and existing tests can be reused with trivial adjustments.
[shmemfix2] pass DataSourceClient to ZoneDataLoader, not just an iterator.

ZoneDataLoader now creates an iterator internally.  we'll then be able to
create and use a journal reader for diff-based loading.
[shmemfix2] refactor: use ZoneDataLoader from zone writer.

some adjustment was needed, but should mostly be straightforward.
[shmemfix2] cleanup: removed unused file.
[shmemfix2/2913] cleanup: renamed load_action.h to reflect its new purpose.

this completes Trac2913, although commit() method is not yet implemented
and obviously unused.
[shmemfix2] cleanup: handle null from getIterator() in ZoneDataLoader.

some test cases are not valid anymore and can simply be removed.
[shmemfix2] supported optimization of skipping load for the same serial.
[shmemfix2] added a few informational/debug log messages.
[shmemfix2/2860] use skip-load optimization of ZoneDataLoader from writer.

this completes trac ticket 2860.  it works with memmgr without further changes.
[shmemfix2] editorial cleanup: folded a long line due to isc->bundy
[shmemfix2] add findName/removeNode to ZoneData and NSEC3Data

these small extensions will be needed to implement ZoneDataUpdater::remove().
[shmemfix2] clarified RdataSet::subtract returns NULL if the result is empty.
[shmemfix2/2752] added ZoneDataUpdater::remove() for diff based zone load

this completes trac ticket 2752.  note that the method is named 'remove',
as 'delete' is reserved for C++.
[shmemfix2] don't attempt removing origin nodes; domaintree doesn't check it.
[shmemfix2] added a few more test cases for ZoneData::remove
[shmemfix2] added log message for removing RRs
[shmemfix2] added ZoneDataLoader::commit() to support diff-based load.
[shmemfix2] updated ZoneWriter so it uses ZoneDataLoader::commit().

this will enable diff-based loading at the application level.
[shmemfix2] added some more comments
[shmemfix2/2912] fix a minor case of becoming NSEC3 signed to unsigned.

this should complete trac ticket 2912 and other tickets for diff/jnl
based zone loading.