- validate consistent checksum settings
    - Make sure checksum settings for all segments are same as master
    - Add logging proxy to allow logging to file to have different contents
      than stdout
    - Do heap checksum only for when starting up all segments.

- Add option --skip-heap-checksum-validation
    - If this option is provided to gpstart, the cluster will start up without
      checking for matching "data_checksums" GUC between master and segments.
Signed-off-by: NNadeem Ghani <nghani@pivotal.io>
Signed-off-by: NMarbin Tan <mtan@pivotal.io>