Refactor queue to make it easier to customize behavior

[adam-26]

Refer to issue #947.

For an example usage, I have created a 'topical queue', where each queue item is assigned a topic. Each topic can be paused/resumed without impacting other topics. See here: adam-26@6ba827d

[aearly]

I like that this simplifies the implementation of queue a bit, but I don't really like how it exposes some of the private internals (e.g. q._getTasksToProcess). If we expose them, we'll need tests for them, and should make them public. Otherwise, if we refactor the queue some more (which is likely), we'll inadvertently break people's code.