ProcessBookkeepingService encode logged items for ES ingestion

What does this MR do?

Elasticsearch ingestion won't be able to handle an array of different types and thus the log entry will be blocked. As such we can't find these log statements in Kibana at the moment. Changing to just a string will allow it to be ingested properly.

Screenshots

Example logs:

{"severity":"DEBUG","time":"2020-03-23T02:02:35.525Z","correlation_id":"FK5k2Sq4OHa","message":"track_items","count":1,"tracked_items_encoded":"[[1,\"Note 985 note_985 project_18\"]]"}
{"severity":"DEBUG","time":"2020-03-23T02:02:35.637Z","correlation_id":"FK5k2Sq4OHa","message":"track_items","count":1,"tracked_items_encoded":"[[2,\"Issue 333 issue_333 project_18\"]]"}

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team