refactor: Split dbwrapper CDBWrapper::Exists 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

@@ -313,6 +313,21 @@ std::optional<std::string> CDBWrapper::ReadImpl(Span<const std::byte> ssKey) con
     return strValue;
 }
 
+bool CDBWrapper::ExistsImpl(Span<const std::byte> ssKey) const
+{
+    leveldb::Slice slKey(CharCast(ssKey.data()), ssKey.size());
+
+    std::string strValue;
+    leveldb::Status status = pdb->Get(readoptions, slKey, &strValue);
+    if (!status.ok()) {
+        if (status.IsNotFound())
+            return false;
+        LogPrintf("LevelDB read failure: %s\n", status.ToString());
+        dbwrapper_private::HandleError(status);
+    }
+    return true;
+}
+
 bool CDBWrapper::IsEmpty()
 {
     std::unique_ptr<CDBIterator> it(NewIterator());