[BUG] Concurrently updating list field in a sample from different processes does not keep all changes

[brimoor]

Due to the same underlying reason explained in #3185, concurrently updating a list field of a sample held in-memory in multiple processes can cause data loss.

Note that this is only an issue if a sample object is held in-memory in one process while being edited in another process, which is not as common as #3185.

Ideally this would be fixed by Document._save() intelligently performing array updates using $push, $pull, etc rather than $set-ing the entire field:

fiftyone/fiftyone/core/odm/document.py

Line 671 in 3026474

sets, unsets = self._delta()

But unfortunately mongoengine does not natively support this:
https://docs.mongoengine.org/guide/document-instances.html#saving-and-deleting-documents

TERMINAL A

import fiftyone as fo
import fiftyone.zoo as foz

# step 1
ds = foz.load_zoo_dataset('quickstart', max_samples=2)
ds.persistent = True

# step 2
s = ds.first()
s['listfld'] = ['a']
s.save()

# step 5
s['listfld'] += ['c','cc']

# step 7
s.save()

# step 9
ds.reload()

print(ds.first())
# Output: listfld contains ['a','c','cc']

TERMINAL B

import fiftyone as fo

# step 3
ds = fo.load_dataset('quickstart-2')

# step 4
s = ds.first()
s['listfld'] += ['b','bb']

# step 6
s.save()

# step 8
ds.reload()

print(ds.first())
# Output: listfld contains ['a','c','cc']