Flushes a previously created block of messages created with BeginUpdateSession. Notice that BeginUpdateSession may be called more than once, and for each BeginUpdateSession called, an EndUpdateSession call must be made; when this happens, the data will be flushed over the network, and all the messages will be send in one virtual packet (conserving both speed, and bandwidth).
Return value:
Returns the amount of data flushed (contained by the cache buffer), or 0 if the data was not cached (no BeginUpdateSession was called, or no data was cached).