Refactor low-level Source operations for better reuse

[sdboyer]

There's a lot of inter-calls on baseVCSSource, and its derived types, as they exist today. These types have seen...uh, let's call it "organic growth," and are turning into spaghetti. The main problems are:

• Pretty much all of the methods take out locks - both the "porcelain" and the "plumbing." As a result, porcelain often ends up reimplementing plumbing, as calling the plumbing would deadlock.
• There's a proliferation of individual bits, flags, bools, etc., which represent separate but interrelated states. These need to be consolidated and managed better.

Basically, I got lazy and started focusing on actions, rather than retaining the overall-consistent-state approach, and let things fall into disrepair.

[sdboyer]

This is fixed by #196