dbwrapper: reuse iterator scratch stream

`CDBIterator::Seek()` and `CDBIterator::GetValue()` only need a temporary owning buffer for immediate use.

Keep one preallocated `DataStream` scratch member on the non-copyable iterator and guard each use with `ScopedDataStreamUsage`.
`SeekImpl()` consumes the serialized key during the immediate LevelDB seek, and `GetValue()` still copies LevelDB value bytes into the owning scratch stream before in-place deobfuscation.

A single stream is enough because these methods cannot overlap on the same iterator without re-entering or concurrently using it, which the guard asserts against.
The preceding test covers repeated seeks and failed value deserialization followed by a successful read from the same entry.

Co-authored-by: Andrew Toth <andrewstoth@gmail.com>

src/dbwrapper.cpp

@@ -360,7 +360,10 @@ struct CDBIterator::IteratorImpl {
 };
 
 CDBIterator::CDBIterator(const CDBWrapper& _parent, std::unique_ptr<IteratorImpl> _piter) : parent(_parent),
-                                                                                            m_impl_iter(std::move(_piter)) {}
+                                                                                            m_impl_iter(std::move(_piter))
+{
+    m_scratch.reserve(DBWRAPPER_PREALLOC_KEY_SIZE);
+}
 
 CDBIterator* CDBWrapper::NewIterator()
 {