Handle moveDistinctIds race conditions

[yakkomajuri]

Changes

Me and @macobo will probably discuss this a bit further tomorrow.

Handles 2 types of race conditions that may arise when running mergeDistinctIds:

• mergeInto person no longer exists: this triggers an error and leaves the persons unmerged
• otherPerson no longer exists: this currently does not trigger an error but leaves persons unmerged

These happen because in between the time the persons are fetched and moveDistinctIds is called, another thread may have deleted either of the persons. Happy to explain further.

Checklist

• [ ] Updated Settings section in README.md, if settings are affected
• [ ] Jest tests

[yakkomajuri]

Changing the approach from the "inline alias" required adding totalMergeAttempts to alias so we can bail out of what could under very rare and specific circumstances be an infinite loop. Also not super clean :D

Also had moved towards moveDistinctIds returning a success status but I think the explicit RaceConditionError makes sense here

[yakkomajuri]

@macobo this should be good for a second look

[yakkomajuri]

I really should put some effort into deflaking these tests 😠