fix: Flush buffers before renaming cache file (#2927)

If clio shuts itself down due to exceeding graceful period when cache is
saved and renamed but the buffers are not flushed, we may end up with a
corrupted cache file. Clio will detect corruption and will not load
corrupted cache file, but we could avoid it by explicitly flushing
ofstream buffer.

src/data/impl/OutputFile.hpp

@@ -25,6 +25,7 @@
 
 #include <cstddef>
 #include <cstring>
+#include <expected>
 #include <fstream>
 #include <string>
 
@@ -60,6 +61,9 @@ public:
     ripple::uint256
     hash() const;
 
+    std::expected<void, std::string>
+    close();
+
 private:
     void
     writeToFile(char const* data, size_t size);