Due to the asynchronous nature of `ldms_stream`, a stream event may
already in the process of being delivered to the application callback
function at the same time that the application closes the stream client
and releases application's resources associated with it. This results in
a use-after-free.

This patch adds `LDMS_STREAM_EVENT_CLOSE` callback event that is
delivered to the application callback function when the stream client is
closed. The `LDMS_STREAM_EVENT_CLOSE` is the last event to be delivered
to the client call back function, so that the application can then
safely release resources asscoiated with the stream client after
receiving it.