Add the current model memory limit and the number of bytes in
excess of that at the point of the last allocation failure to the model
size stats. These will be used to construct a (hopefully) more
informative hard_limit audit message.

The reported memory usage is also scaled to take into account the byte
limit margin, which is in play in the initial period of a jobs' lifetime
and is used to scale down the high memory limit. This should give a more
accurate representation of how close the memory usage is to the high
limit.

relates elastic/elasticsearch#42086

closes elastic/elasticsearch#38034