add optional per-queue environment value

[bwlewis]

Per-job, foreach shared values (either automatically exported or explicitly exported with .export=...) are guaranteed to be on the workers and only copied once to them even if they take multiple tasks for a given job. This is accomplished by doRedis with a single shared per-job key/value.

We could similarly add a global per-queue shared key/value containing whatever stuff you want guaranteed to be on each worker connected to a given queue. This should be easy to add.