issue-368: unmap free spans and retry before growing heap
issue-368: unmap free spans and retry before growing heap

Because unmapped spans are not coalesced with normal spans it's
possible that we indeed have a large enough free span, but we fail to
see that because we always consider unmapped and normal spans
separately. That behavior is more likely for larger spans.

In order to protect programs that grow heap frequently and by small
amounts from much more frequent minor page faults, there's limit of
running that force pages unmap path once per 128 megs of heap growth.



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