Implement persistent caching in SourceMgr

[sdboyer]

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With additional testing being done as part of the new tool, it's increasingly clear that we really, really need to get persistent, disk-backed caching into place. There are four basic things we need to cache:

2. Version lists - all the versions available for a source
3. Manifest and Lock - for a given rev and a particular analyzer, these should be permanently cacheable
4. PackageTree - for a given rev and gps version, these should be permanently cacheable
5. Project root computations - those that require a go-get metadata query

The biggest immediate question is how to actually store it on disk. For the first three, at least, discrete files would probably work. However, I'm inclined to store everything together using bolt. That'll cover the last case more easily, especially because it is likely to require prefix queries; it'll also probably make it a bit cleaner to store TTLs.

(supercedes #8)

[sdboyer]

With #196 finally done, the road to this is pretty clear. There's even a TODO exactly where a handle to it should be dropped in 😄

We do need to pick a storage lib for it. LevelDB would be fine, as would Bolt.

[fabulous-gopher]

This issue was moved to golang/dep#431