Fix MongoStore `update` speed

[munrojm]

Example: Using my_store._collection.insert_many() -> 4.34 seconds Using my_store.update() -> 279.79 seconds.

update is doing a bulk_write of ReplaceOne requests (perhaps pointing to a missing index on the collection I’m inserting to?), but it does probably point to needing an alternative method when we know that we’re inserting to an empty collection.

[rkingsbury]

See also related question in #289