Refactor SourceMgr to eliminate concurrency problems, be generally sane

[sdboyer]

Right now, the SourceMgr implementation (and its interaction with the contained source classes) is pretty messy. There is certainly some bad concurrency handling (as evidenced by intermittent failures of TestMultiDeduceThreadSafe()), and some race conditions. And, more generally, it's verging on spaghetti. It really needs refactoring.

The new model, I think, should make aggressive use of channels to manage the flow of calls and data through the system. It'd make it far easier to serialize and control access to segments where it's necessary, to offer the sort of control that would enable cancellation of calls (#159), and provide the sort of layering that would allow us to implement the needed on-disk caching (#130).