[heapstat] combine heapstat data for all child processes

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 19:52:12_

PR 469013

we can combine the data from all the child processes of an app
using timestamps recorded with the callstack data. xref PR 465163, though
here we'll want timestamps to go with each allocation.

perhaps we can use the same aggregation to combine data from multiple nudges to show a range of nudges

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=209_