An archive library is loaded by dynamically loading a shared library with the
   archive_library's name as the library base name.  The
   normal library search path is used to locate the library.  To provide the
   required archive module callbacks and to indicate that the library is
   actually an archive module, it needs to provide a function named
   _PG_archive_module_init.  This function is passed a
   struct that needs to be filled with the callback function pointers for
   individual actions.
typedef struct ArchiveModuleCallbacks
{
    ArchiveCheckConfiguredCB check_configured_cb;
    ArchiveFileCB archive_file_cb;
    ArchiveShutdownCB shutdown_cb;
} ArchiveModuleCallbacks;
typedef void (*ArchiveModuleInit) (struct ArchiveModuleCallbacks *cb);
   Only the archive_file_cb callback is required.  The
   others are optional.