Add support for Bulk Write into multiple collections.

[christophstrobl]

See: https://www.mongodb.com/docs/manual/core/bulk-write-operations/#mongo.bulkwrite--

The BulkOperations interface itself does not define on which collection the operation is performed. So we could keep the interface exposed backing it with an collection agnostic implementation, or change the current DefaultBulkOperations so that it handles the extended use case.