[BUG] Concurrently adding new fields from different processes is unstable

[brimoor]

Executing the steps below in the order indicated raises errors on fiftyone<0.22.

The root problem is multiple processes adding elements to DatasetDocument.sample_fields concurrently. Rather than both adding elements using array updates, they are both be $set-ing the entire field, as mongoengine does not support intelligent granular updates of array fields.

TERMINAL A

import fiftyone as fo
import fiftyone.zoo as foz

# step 1
dataset = foz.load_zoo_dataset("quickstart", dataset_name="zzz")
dataset.persistent = True

# step 3
sample = dataset.first()
sample["foo"] = "bar"

# step 5
sample.save()

# step 7: ERROR
dataset.reload()
# FieldDoesNotExist: The fields "{'spam'}" do not exist on the document "samples.XXXXXXXX"

TERMINAL B

import fiftyone as fo

# step 2
dataset = fo.load_dataset("zzz")

# step 4
sample = dataset.first()
sample["spam"] = "eggs"

# step 6
sample.save()

# step 8: ERROR
dataset.reload()
# FieldDoesNotExist: The fields "{'foo'}" do not exist on the document "samples.XXXXXXXX"