Freelist of buffers is frequently needed to avoid GC/zeroing overhead in
e.g. networked servers. Making buffer reference-counted is then required
if buffer is shared between several users so that it is clear when it
can go back to its pool. In ZODB this situation was arising on returning
the same buffer from cached loads if several loads for same data are
issued in close to parallel.

[1] https://lab.nexedi.com/kirr/neo/tree/75a71514/go/zodb