ZOID does not work with buffers in flat memory

[iskra]

If flat memory is used (buffers_flat.c), ZOID becomes very unreliable. In particular, IP forwarding seems to be completely broken, but I/O forwarding is not exactly reliable, either (rawiobench experiments with 64 clients were unsuccessful).

Debugging of IP forwarding problem indicates that data is correctly written into the zoid buffer by the IP forwarding thread, but the worker thread writing that data to the tree sees zeros in the same memory locations! We do pthread_mutex_lock/unlock, which should do memory barrier, so we should be OK.