Introduce ZSTD_getFrameProgression()

Introduce ZSTD_getFrameProgression()

Produces 3 statistics for ongoing frame compression :
- ingested
- consumed (effectively compressed)
- produced

Ingested can be larger than consumed due to buffering effect.

For the time being, this patch mostly fixes the % ratio issue,
since it computes consumed / produced,
instead of ingested / produced.

That being said, update is not "smooth",
because on a slow enough setting,
fileio spends most of its time waiting for a worker to complete its job.

This could be improved thanks to more granular flushing
i.e. start flushing before ongoing job is fully completed.