DRTVWR-476, SL-13512: Make suspendUntilTimeout() notice shutdown.

Specifically, the shutdown crash reported in SL-13512 was due to
LLExperienceCache::idleCoro() looping on suspendUntilTimeout(), failing to
notice in its slumbers that the viewer was shutting down around it.

Make suspendUntilTimeout() internally call suspendUntilEventOnWithTimeout(),
which already listens for "LLApp" state-change events and throws Stopping when
LLApp enters its shutdown sequence.