Merge #1287: NFT db records optimizations

Pull request description:

  * Adding an iterator wrapper class
  * Prevent double lookups
  * Change lookups from O(n) to log O(n)

src/omnicore/dbbase.cpp

@@ -33,15 +33,13 @@ void CDBBase::Clear()
     int64_t nTimeStart = GetTimeMicros();
     unsigned int n = 0;
     leveldb::WriteBatch batch;
-    leveldb::Iterator* it = NewIterator();
+    CDBaseIterator it{NewIterator()};
 
-    for (it->SeekToFirst(); it->Valid(); it->Next()) {
+    for (; it; ++it) {
         batch.Delete(it->key());
         ++n;
     }
 
-    delete it;
-
     leveldb::Status status = pdb->Write(writeoptions, &batch);
     nRead = 0;
     nWritten = 0;

src/omnicore/dbbase.h

@@ -1,11 +1,13 @@
 #ifndef BITCOIN_OMNICORE_DBBASE_H
 #define BITCOIN_OMNICORE_DBBASE_H
 
+#include <algorithm>
 #include <leveldb/db.h>
 
 #include <fs.h>
 
 #include <assert.h>
+#include <memory>
 #include <stddef.h>
 
 /** Base class for LevelDB based storage.
@@ -93,5 +95,48 @@ class CDBBase
     void Clear();
 };
 
+class CDBaseIterator
+{
+private:
+    std::unique_ptr<leveldb::Iterator> it;
+
+public:
+    explicit CDBaseIterator(leveldb::Iterator* i, const std::string& first = {}) : it(i)
+    {
+        assert(it);
+        first.empty() ? it->SeekToFirst() : it->Seek(first);
+    }
+
+    CDBaseIterator& operator=(CDBaseIterator&& i)
+    {
+        it = std::move(i.it);
+        assert(it);
+        return *this;
+    }
+
+    CDBaseIterator& operator++()
+    {
+        assert(it->Valid());
+        it->Next();
+        return *this;
+    }
+
+    CDBaseIterator& operator--()
+    {
+        assert(it->Valid());
+        it->Prev();
+        return *this;
+    }
+
+    operator bool() const
+    {
+        return it->Valid();
+    }
+
+    leveldb::Iterator* operator->()
+    {
+        return it.get();
+    }
+};
 
 #endif // BITCOIN_OMNICORE_DBBASE_H