AsyncFileSystem regression: _makedirs

[isidentical]

#686 introduced a requirement over _makedirs() of the downstream filesystems, which is something optional according to this list;
https://github.com/intake/filesystem_spec/blob/ce0b70320d5d8704a97aafa45b17b8e60bd8ecd5/fsspec/asyn.py#L210-L230

I think there should be empty methods _mkdir/_makedirs that does nothing:
https://github.com/intake/filesystem_spec/blob/ce0b70320d5d8704a97aafa45b17b8e60bd8ecd5/fsspec/spec.py#L264-L296