Make BufferedFile to be a CAutoFile wrapper

This refactor allows to forward some calls to the underlying CAutoFile,
instead of re-implementing the logic in the buffered file.

src/validation.h

@@ -607,7 +607,6 @@ public:
     bool ResizeCoinsCaches(size_t coinstip_size, size_t coinsdb_size)
         EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
-
     /**
      * Update the on-disk chain state.
      * The caches and indexes are flushed depending on the mode we're called with
@@ -1087,14 +1086,14 @@ public:
      * -loadblock= option. There's no unknown-parent tracking, so the last two arguments are omitted.
      *
      *
-     * @param[in]     fileIn                        FILE handle to file containing blocks to read
+     * @param[in]     file_in                       File containing blocks to read
      * @param[in]     dbp                           (optional) Disk block position (only for reindex)
      * @param[in,out] blocks_with_unknown_parent    (optional) Map of disk positions for blocks with
      *                                              unknown parent, key is parent block hash
      *                                              (only used for reindex)
      * */
     void LoadExternalBlockFile(
-        FILE* fileIn,
+        CAutoFile& file_in,
         FlatFilePos* dbp = nullptr,
         std::multimap<uint256, FlatFilePos>* blocks_with_unknown_parent = nullptr);