When WT_CURSOR cursors are cached, we free their key/value buffers. However, those cursors are likely WT_CURSOR_BTREE cursors underneath, which hold up to 4 additional buffers, which should likely be freed as well.

We could discard those buffers when WT_CURSOR.reset is called, but that may no be a good idea, given the frequency at which MongoDB server calls that function.

Alternatively, we could add a new Btree level cache function.