refactor: Split dbwrapper CDBBatch::Write 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

@@ -6,6 +6,8 @@
 
 #include <logging.h>
 #include <random.h>
+#include <span.h>
+#include <streams.h>
 #include <tinyformat.h>
 #include <util/fs.h>
 #include <util/fs_helpers.h>
@@ -23,7 +25,9 @@
 #include <leveldb/helpers/memenv/memenv.h>
 #include <leveldb/iterator.h>
 #include <leveldb/options.h>
+#include <leveldb/slice.h>
 #include <leveldb/status.h>
+#include <leveldb/write_batch.h>
 #include <memory>
 #include <optional>
 
@@ -132,6 +136,22 @@ static leveldb::Options GetOptions(size_t nCacheSize)
     return options;
 }
 
+void CDBBatch::WriteImpl(Span<const std::byte> ssKey, CDataStream& ssValue)
+{
+    leveldb::Slice slKey(CharCast(ssKey.data()), ssKey.size());
+    ssValue.Xor(dbwrapper_private::GetObfuscateKey(parent));
+    leveldb::Slice slValue(CharCast(ssValue.data()), ssValue.size());
+    batch.Put(slKey, slValue);
+    // LevelDB serializes writes as:
+    // - byte: header
+    // - varint: key length (1 byte up to 127B, 2 bytes up to 16383B, ...)
+    // - byte[]: key
+    // - varint: value length
+    // - byte[]: value
+    // The formula below assumes the key and value are both less than 16k.
+    size_estimate += 3 + (slKey.size() > 127) + slKey.size() + (slValue.size() > 127) + slValue.size();
+}
+
 CDBWrapper::CDBWrapper(const DBParams& params)
     : m_name{fs::PathToString(params.path.stem())}, m_path{params.path}, m_is_memory{params.memory_only}
 {