[WIP] Allow reloading database connection

[brimoor]

Marked as draft for discussion.

Inspired by a Slack community question, I wanted to see what it would look like to allow changing databases mid-Python session by calling a fo.reload() method.

I believe this PR is fully functional for programmatic usage, but there are a couple snags when using the App:

• The App runs in a subprocess which currently re-reads its fo.config from environment variables, so you have to change database via os.environ rather than just editing fo.config in order for launch_app() to work after the database change in the example below
• If I launch_app() before and after the database change, the second App does not connect to the new database

import os
import fiftyone as fo
import fiftyone.zoo as foz

# do stuff with default database
print(fo.list_datasets())

#
# Change database
#

"""
os.environ["FIFTYONE_DATABASE_URI"] = "mongodb://localhost:12345"
os.environ["FIFTYONE_DATABASE_DIR"] = "/path/to/new/db"
fo.reload(hard=True)
"""

fo.config.database_uri = "mongodb://localhost:12345"
fo.config.database_dir = "/path/to/new/db"
fo.reload()

# do stuff with new database
print(fo.list_datasets())
dataset = foz.load_zoo_dataset("quickstart")

# Only works with `fo.reload(hard=True)` since subprocess reads config from env vars
session = fo.launch_app(dataset)

[codecov]

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[kaixi-wang]

unsure how/if changing the mongo connection would affect pymongo, but through aliased connections (https://docs.mongoengine.org/guide/connecting.html#switch-database), you could maybe hack a way to overcome the snags you mentioned?