refactor: Use uint64_t over size_t for serialized-size values

The values are small enough to fit in size_t, but to avoid having to
think about it, just use uint64_t consistently for all architectures.

On 64-bit systems, this refactor is a no-op. On 32-bit systems, it could
avoid bugs in the theoretical and unexpected case where a 32-bit size_t
is too small and overflows.

src/index/blockfilterindex.cpp

@@ -202,9 +202,9 @@ size_t BlockFilterIndex::WriteFilterToDisk(FlatFilePos& pos, const BlockFilter&
 {
     assert(filter.GetFilterType() == GetFilterType());
 
-    size_t data_size =
+    uint64_t data_size{
         GetSerializeSize(filter.GetBlockHash()) +
-        GetSerializeSize(filter.GetEncodedFilter());
+        GetSerializeSize(filter.GetEncodedFilter())};
 
     // If writing the filter would overflow the file, flush and move to the next one.
     if (pos.nPos + data_size > MAX_FLTR_FILE_SIZE) {