Page Comparison

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1. TransformContext changes
   1. Rename TransformContext to StageContext (since it is used as the base context in BatchRuntimeContext, not only transform contexts).
   2. Create TransformContext which extends StageContext
      1. Has a single method: Lookup getLookup()
2. Transform changes
   1. Lookup interface with lookup methods that perform read operations on datasets
   2. DefaultLookup: implementation that uses Transactional to implement Lookup, used in ETLWorker
   3. Add Lookup field to ScriptContext, so Lookup is accessible via transforms that interpret JavaScript
      1. Sample usage: ctx.lookup.lookupKVString('purchases', 'key')

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