* Refactor Loader from AnalysisAdapter
At present, the only data the AnalysisAdapter provides is the Graph, so
the AnalysisAdapter has a `load` method which directly returns the
graph. I'm planning to add a `createdAt` getter to the adapter as well,
which also will depend on loading the data. To make this change
convenient, I'm starting by refactoring an AdapterLoader out, which
manages loading data from disk, so that once we have an AnalysisAdapter,
it already has all relevant data loaded. Then, it will be much easier to
add a `createdAt` method.
Test plan: Tests updated, flow passes.
* Add `createdAt` to the analysis adapter
A big step forward for time-varying cred. This will make `createdAt`
timestamps available for PageRank analysis.
Test plan: Added new unit tests. Inspect the snapshots. Run `yarn test`.
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