Each client connection that is accepted by the server is assigned
a unique numeric value. This value can be used by the application to
identify that client session, and is different than the socket handle
allocated for the client. While it is possible for a client socket
handle to be reused by the operating system, client IDs are unique
throughout the life of the server session and are never
duplicated.
It is important to note that the actual value of the client ID
should be considered opaque. It is only guaranteed that the value
will be greater than zero, and that it will be unique to the client
session.
This property only returns a meaningful value when accessed from
within an event handler, or a function that has been called from
within an event handler.