Add pool name option to worker pools

[F-3r]

I found myself debugging an error in production infrastructure that made some of the DJ workers to die (or not start at all) and I was having a hard time figuring out which of the pool where missing workers.
This PR adds a 3rd optional parameter to the pool specification syntax to add a tag to the processes name that indicates the pool they belong to.

The syntax is --pool=queue_1:30:a-pool-name this will spawn 30 processes named delayed_job.a-pool-name.{0..29}.

If omitted, then it uses the standard naming.