Feature request: Worker port ranges and/with nprocs support

I think it would be quite useful for dask-worker to support --nprocs with --foo-port=[range] so that systems with specific port ranges opened can have a number of processes opened from one call applied to a specified range opposed to having to write a shell loop to do the same.

Further, general support for port ranges and failing over to the next available port in a range if the current is taken would probably be quite useful in a similar situation as that described above.

Thanks.

[stuartarchibald]

Thanks for implementing this @jrbourbeau!

[stuartarchibald]

I think being able to express a port range for each component would be ideal thanks. Something like:

dask-worker --nprocs=16 --worker-port=5000:5100 --http-port=6000:6100

which would start 16 worker processes bound to free ports between 5000 and 5100 and the same with http but s/5/6/g. This is with view of use cases where open ports are restricted to some particular range but some external reason requires computation to be undertaken in separate processes.