validation: Refactor file flush logic into FlatFileSeq.

src/flatfile.h

@@ -45,6 +45,15 @@ public:
      * @return The number of bytes successfully allocated.
      */
     size_t Allocate(const CDiskBlockPos& pos, size_t add_size, bool& out_of_space);
+
+    /**
+     * Commit a file to disk, and optionally truncate off extra pre-allocated bytes if final.
+     *
+     * @param[in] pos The first unwritten position in the file to be flushed.
+     * @param[in] finalize True if no more data will be written to this file.
+     * @return true on success, false on failure.
+     */
+    bool Flush(const CDiskBlockPos& pos, bool finalize = false);
 };
 
 #endif // BITCOIN_FLATFILE_H