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/bench/load_external.cpp

@@ -56,7 +56,7 @@ static void LoadExternalBlockFile(benchmark::Bench& bench)
         // "rb" is "binary, O_RDONLY", positioned to the start of the file.
         // The file will be closed by LoadExternalBlockFile().
         CAutoFile file{fsbridge::fopen(blkfile, "rb"), CLIENT_VERSION};
-        testing_setup->m_node.chainman->LoadExternalBlockFile(file.Get(), &pos, &blocks_with_unknown_parent);
+        testing_setup->m_node.chainman->LoadExternalBlockFile(file, &pos, &blocks_with_unknown_parent);
     });
     fs::remove(blkfile);
 }

src/bench/streams_findbyte.cpp

@@ -20,7 +20,7 @@ static void FindByte(benchmark::Bench& bench)
     data[file_size-1] = 1;
     file << data;
     std::rewind(file.Get());
-    BufferedFile bf{file.Get(), /*nBufSize=*/file_size + 1, /*nRewindIn=*/file_size, 0};
+    BufferedFile bf{file, /*nBufSize=*/file_size + 1, /*nRewindIn=*/file_size};
 
     bench.run([&] {
         bf.SetPos(0);