refactor: Split dbwrapper CDBIterator::GetKey implementation

Keep the generic serialization in the header, while moving
leveldb-specifics to the implementation file.

The context of this commit is an effort to decouple the dbwrapper header
file from leveldb includes. To this end, the includes are moved to the
dbwrapper implementation file. This is done as part of the kernel
project to reduce the number of required includes for users of the
kernel.

src/dbwrapper.cpp

@@ -312,6 +312,11 @@ void CDBIterator::SeekImpl(Span<const std::byte> ssKey)
     piter->Seek(slKey);
 }
 
+Span<const std::byte> CDBIterator::GetKeyImpl() const
+{
+    return MakeByteSpan(piter->key());
+}
+
 CDBIterator::~CDBIterator() { delete piter; }
 bool CDBIterator::Valid() const { return piter->Valid(); }
 void CDBIterator::SeekToFirst() { piter->SeekToFirst(); }

src/dbwrapper.h

@@ -141,6 +141,7 @@ private:
     leveldb::Iterator *piter;
 
     void SeekImpl(Span<const std::byte> ssKey);
+    Span<const std::byte> GetKeyImpl() const;
 
 public:
 
@@ -166,9 +167,8 @@ public:
     void Next();
 
     template<typename K> bool GetKey(K& key) {
-        leveldb::Slice slKey = piter->key();
         try {
-            DataStream ssKey{MakeByteSpan(slKey)};
+            DataStream ssKey{GetKeyImpl()};
             ssKey >> key;
         } catch (const std::exception&) {
             return false;