Fix refleak tracking in free-threaded build

[colesbury]

Bug report

There are a few bugs with refleak tracking in the free-threaded build uncovered in #114824:

• We should account for blocks in abandoned segments
• We should stop-the-world before traversing mimalloc heaps in case there are still other threads running
• The mimalloc heap and segment traversal must call _mi_page_free_collect(page, true); to properly account for blocks that were deferred freed by other threads. We need to call this earlier than we currently do (before computing page stats).
• _Py_DecRefSharedDebug was missing a _Py_IncRefTotal

Linked PRs

• gh-115184: Fix refleak tracking issues in free-threaded build #115188