The onDisconnect class allows you to write or clear data when your client
disconnects from the Database server. 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 sorts and filters the data at a Database location so only a subset
of the child data is included. This can be used to order a collection of
data by some attribute (for example, height of dinosaurs) as well as to
restrict a large list of items (for example, chat messages) down to a number
suitable for synchronizing to the client. Queries are created by chaining
together one or more of the filter methods defined here.
