Rework CNameDB public interface.

Rework the public interface of CNameDB to be more high-level.  Instead
of reading (and especially writing) the vector of CNameIndex objects
manually, users of CNameDB can now use PushEntry/PopEntry and ReadName
to access just the last entry.  This is in preparation to pruning.
Simplify name_list (no need for CNameDB).

Use the Merkle branch info to determine a tx's height in name_list, no
need to use the name DB.
Do name_list optimisations also for Qt.
Fix the name_list improvements.
Remove suprefluouns conditional.
Merge pull request #76 from domob1812/prune-namelist

Encapsulate CNameDB interface and improve name_list.