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(1) A ExamplePreprocessorConfig proto is added.

(2) A ExamplePreprocessor class that build on a config proto and does
    preprocessing including missing feature handling and bucketization.

(3) A ExampleFloatIterator is also added to iterate through all
    features of a RankerExample.

(4) Add a unittest.

Change-Id: I0de0f2fa6d6e9b7ae3b873d8b7b00009cd04ec80
Reviewed-on: https://chromium-review.googlesource.com/872831Reviewed-by: Philippe Hamel <hamelphi@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Charles . <charleszhao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532089}