cls/fifo: Fix race condition on `_prepare_new_head`

cls/fifo: Fix race condition on `_prepare_new_head`

First, make `_prepare_new_head` take the new head part number, so two
calls racing from the same push will attempt to create the same head.

Also remove the neorados FIFO since it doesn't have all the bug fixes
in the legacy version and will be rewritten in terms of `async_compose`
anyway.

Fixes: https://tracker.ceph.com/issues/57562
Signed-off-by: Adam C. Emerson <aemerson@redhat.com>