chapter303, [SORA][FinexDriveChain] implement CLevelDB::const_iterator

The code that handles LevelDB has been greatly improved.
After improvement it as finely as possible, we implemented the iterator.

After calling to the seek method,
we can now search LevelDB in the usual way by iterator.
And, memory management is auto.

seek(KEY, VALUE);
for(const_iterator iterator = begin(); iterator != end(); ++iterator) { statement }

db.h
https://github.com/FromHDDtoSSD/SorachanCoin-qt/blob/master/src/db.h