Fixups for poisson rate

Fixups for poisson rate

- Change name to poisson_rate, we could add other uses for this
  functionality (like depth, etc).

- Add "rate" as option parent.

- Ensure we convert the option for client/server, bump protocol
  version.

- Add a separate rand state for poisson, so it becomes randomly
  controllable (through randrepeat).

- Add random 0..1 helper.

Signed-off-by: Jens Axboe <axboe@fb.com>