Evtx sessionizer

[katemacleod]

Replaces previous pull request #964 (changes to tests needed to be made for compatibility with other changes. These tests would be overwritten by this pull request anyway). Solution for issues #955 and #952

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[tomchop]

Manually setting CLA to yes because we rebased from the GitHub UI

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[tomchop]

Changed a few style nits, otherwise LGTM