Lrge allocation threshold never goes down

[xemul]

It's designed like that, but it has a side effect -- rare large allocations may shadow more interesting smaller ones forever. E.g. -- if startup allocates large chunk of memory, it's boring, but due to this we can skip some more interesting one.

Need a timely fall of the threshold, say, once every severla minutes.

[avikivity]

Startup disables large allocation reporting, but the rest of it holds.

[xemul]

Dup of #669