Reimplement ExportOp in a streamed fashion.

Instead of building an in-memory FeatureCollection to then be exported, use
guava's utilities to create a FeatureCollection that relies in a transforming
iterator that traverses the features in a RevTree and applied the operation
filters/transformations on the fly.
Merge pull request #419 from groldan/issue_85_export_oom

Reimplement ExportOp in a streamed fashion.