Reclaim empty overflow slots in memory hash index

[benjaminwinger]

When splitting, move empty overflow slots into a global linked list and re-use them before allocating new slots.

I started working on this to try and simplify how deletions will work in the in memory hash index (needed to unify the hash index local storage for copies and inserts/deletions), as removing empty overflow slots from the end of the slot chains makes it easier to find the last entry in a slot without having to backtrack if the last slot is empty.
This ended up being a little more complicated than I expected, but for a hash index of 60 million consecutive integers it reduces memory use from the slots from roughly 2.28GB to 1.75GB, reducing the number of overflow slots by more than half, and it seems to slightly increase performance (presumably because it reduces the number of allocations).

Something similar could be done for disk slots (see last TODO in #2938 (comment)), the main difference being that disk slots may have gaps, but the gaps could be removed when splitting.

This avoids breaking the storage format by adding the new field (which is not used by the on-disk index) to the end of the hash index header.