windows: trade heap for stack to build process tree for stats in linear space

[tgross]

In #20619 we overhauled how we were gathering stats for Windows processes. Unlike in Linux where we can ask for processes in a cgroup, on Windows we have to make a single expensive syscall to get all the processes and then build the tree ourselves. Our algorithm to do so is recursive and quadratic in both steps and space with the number of processes on the host. For busy hosts this hit the stack limit and panics the Nomad client.

We already build a map of parent PID to PID, so modify this to be a map of parent PID to slice of children and then traverse that tree only from the root we care about (the executor PID). This moves the allocations to the heap but makes the stats gathering linear in steps and space required.

This changeset also moves as much of this code as possible into an area not conditionally-compiled by OS, as the tagged test file was not being run in CI.

Fixes: #23984

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Note to reviewers: I've got 2 commits here. One has the old and new code side-by-side in a test for comparison. The second commit removes the old code. Test results:

$ go test -v -count=1 ./drivers/shared/executor/procstats -run Test_list
=== RUN   Test_list
=== RUN   Test_list/minimal
    list_test.go:120: NEW total: 14 -> examined: 16 (heap: 9192 stack: 0)
    list_test.go:136: OLD total: 14 -> examined: 61 (heap: 7064 stack: 0)
=== RUN   Test_list/small_needles_small_haystack
    list_test.go:120: NEW total: 207 -> examined: 212 (heap: 55784 stack: 0)
    list_test.go:136: OLD total: 207 -> examined: 20112 (heap: 27920 stack: 32768)
=== RUN   Test_list/small_needles_large_haystack
    list_test.go:120: NEW total: 1012 -> examined: 1022 (heap: 288040 stack: 0)
    list_test.go:136: OLD total: 1012 -> examined: 500522 (heap: 150888 stack: 98304)
=== RUN   Test_list/moderate_needles_giant_haystack
    list_test.go:120: NEW total: 2022 -> examined: 2042 (heap: 606408 stack: 0)
    list_test.go:136: OLD total: 2022 -> examined: 2001042 (heap: 332040 stack: 229376)
--- PASS: Test_list (0.32s)
    --- PASS: Test_list/minimal (0.00s)
    --- PASS: Test_list/small_needles_small_haystack (0.01s)
    --- PASS: Test_list/small_needles_large_haystack (0.06s)
    --- PASS: Test_list/moderate_needles_giant_haystack (0.25s)
PASS
ok      github.com/hashicorp/nomad/drivers/shared/executor/procstats    0.353s

[shoenig]

LGTM!