go/zodb: Fix memory leak in ZODB live cache by using weak references

Hello Kirill,

following https://lab.nexedi.com/kirr/neo/-/merge_requests/11#note_247726, this MR fixes the memory leak in ZODB live cache with the new builtin weak references package from go 1.24.

Thanks to your fixes (https://lab.nexedi.com/kirr/neo/-/commit/2382319026c67d7b803c208655b90a3e6caf9d36#note_253577) in go123, all NEO/go tests pass now on my side.

Best, Levin