The Disconnect class encapsulates all operations to be performed on a
Firebase when the client is disconnected. This allows you to write or
clear data when your client disconnects from the Firebase servers. These
updates occur whether your client disconnects cleanly or not, so you can
rely on them to clean up data even if a connection is dropped or a client
crashes.
A Query filters the data at a Firebase location so only a subset of the
child data is visible to you. This can be used for example to restrict a
large list of items down to a number suitable for synchronizing to the
client.
