Prioritized task API

Proposed API for marking queue tasks that should be bumped up to the front.

Needed for Workflow and also useful for [other plugins](https://rm.cloudbees.com/issues/2324), but must be defined in a common layer since there should be a single dispatcher. Considered putting this in a plugin, since it need not be in core, but could not think of anything else that would logically go in the same plugin.

Original idea thanks to @stephenc.