fixup: address PR review comments

- Merge io.h/c back into protocol.h/c
- Change send_data to take Data* argument
- Remove redundant file_load_data from send_chunk
- Move compression into file_send_single_calls
- Move file_receive from multiprocessing.c to file.c

src/shared/metadata.c

@@ -1,6 +1,6 @@
 #include "metadata.h"
 #include "file.h"
-#include "io.h"
+#include "protocol.h"
 #include <fcntl.h>
 #include <stdlib.h>
 #include <string.h>