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
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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