ESC-154 ESC-155 ESC-156

Asset fetch requests wrapped to allow for measurements.
Asset fetch enqueues, dequeues and completion times recorded
to asset stats collector.  Texture fetch operations (http
and udp) recorded to asset stats collector.  Stats collector
time vallue switched from F32 to U64 which is the more common
type in the viewer.  Cross-thread mechanism introduced to
communicate region changes and generate global statistics
messages.  Facility to deliver metrics via Capabilities
sketched in but needs additional work.  Documentation and
diagrams added.