[staleness] store alloc time separately, and set last access time to sentinel on alloc

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 19:52:16_

PR 553710

using alloc time as the initial last access time is a natural way to avoid
any special-casing in the vis tool. using anything else will require extra
vis logic and extra data to store alloc time separately and so essentially
is a separate feature: hence this case.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=220_