Firestore: preserve timestamp nanos in document data

[tseaver]

Uses #6919 as a base: please review only the last commit here until rebased after merging that PR.

Closes #6547.

[tseaver]

The automl failure is a bug in Kokoro, or our setup for it.

error pulling image configuration: Get https://gcr.io/v2/cloud-devrel-kokoro-resources/python-multi/blobs/sha256:bfcdfded6f494ed45ee36a4f28908e3a4b69b9afee63bf0e848bd763cc7bce38: dial tcp 74.125.20.82:443: i/o timeout
Failed to pull docker image, attempt 0 out of 3.
Failed to pull docker image, attempt 1 out of 3.
Failed to pull docker image, attempt 2 out of 3.
Traceback (most recent call last):
  File "/tmpfs/src/gfile/trampoline_v1.py", line 132, in <module>
    main()
  File "/tmpfs/src/gfile/trampoline_v1.py", line 126, in main
    pull_docker_image(image)
  File "/tmpfs/src/gfile/trampoline_v1.py", line 66, in pull_docker_image
    raise RuntimeError("Failed to pull image {}.".format(image))
RuntimeError: Failed to pull image gcr.io/cloud-devrel-kokoro-resources/python-multi.

@crwilcox Failed systests are all due to quota issues in the watch tests.

[mcdonc]

LGTM