vinyl: zap vy_mem_update_formats() + cleanup

vy_mem_update_formats() is used to update mem formats when mem rotation
is skipped, because the mem is empty. This doesn't work as expected,
because vy_mem_update_formats() does not update mem->sc_version, so that
in case of ddl the next insertion will rotate it anyway. Instead of
updating sc_version in vy_mem_update_formats(), let's fix this by
zapping the helper altogether and simply recreating mem - it isn't a big
deal, because this does not happen often. While we are at it, let's
also:
 - reorder arguments of vy_mem_new() to keep key_def close to format
 - remove extra arguments of vy_range_rotate_mem() - we can get all
   of them right there (in fact we already do in case of ->format).