Starts streaming the first part of all streamed files referenced by an Event into a cache buffer. Caching streams are serviced when no other streams require the available bandwidth. The files will remain cached until UnpinEventInStreamCache is called, or a higher priority pinned file needs the space and the limit set by uMaxCachePinnedBytes is exceeded.
||Unique ID of the event
||Priority of active stream caching I/O
||Priority of inactive stream caching I/O