[checkpoint] Change fingerprint creation

[ash-darin]

Enhancement

Proposed commit message

This changes the fields used to create a unique fingerprint for the event. The currently selected fields are lacking in achieving this as rejects have been observed. Checkpoint apparently often generate events that are "updates" to previous events. These events have the timestamp of the original event but incorporate a "lastupdatetime" timestamp and an update_count. Fingerprint creation therefore creates duplicates.

• @timestamp is included, but @timestamp is renamed before use and is therefore always nil. -> removed
• segment_time is not unique, replaced with lastupdatetime
• add update_count

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.

How to test this PR locally

Check your logs/indices if any messages are rejected. Apply fix and check whether numbers go down..