Fixes to savepoint restoration #833
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This makes it more clear that the current freed tree is valid while it is being traversed
Restoring a savepoint dropped processed transactions from the free tree. However, this could lead to pages being leaked, if page A was allocated, then added to the freed tree, then a savepoint was captured, and then later A was freed a allocated again. When the savepoint was restored, A would remain allocated but also be removed from the freed tree
This can't happen, since we only restore at the beginning of a clean transaction
If the file had grown enough that the region tracker had to be reallocated the restoration would free the tracker. This could lead to corruption, if that page was allocated for other use, when the tracker then got flushed to the same location This would only occur in real world usage on very large database files, since the default region tracker is sized for 4TiB
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Fixes #832