Re: Non-reproducible AIO failure

Reproduced it once again with with write-protected io handle.
But once again - no access violation, just assert failure.

Previously "op" field was overwritten somewhere between `pgaio_io_reclaim` and `AsyncReadBuffers`:

!!!pgaio_io_reclaim [20376]| ioh: 0x1019bc000, ioh->op: 0, ioh->generation: 19346
!!!AsyncReadBuffers [20376] (1)| blocknum: 21, ioh: 0x1019bc000, ioh->op: 1, ioh->state: 1, ioh->result: 0, ioh->num_callbacks: 0, ioh->generation: 19346

Now it is overwritten after print in AsyncReadBuffers:

!!!pgaio_io_reclaim [88932]| ioh: 0x105a5c000, ioh->op: 0, ioh->generation: 42848
!!!pgaio_io_acquire_nb[88932]| ioh: 0x105a5c000, ioh->op: 0, ioh->generation: 42848
!!!AsyncReadBuffers [88932] (1)| blocknum: 10, ioh: 0x105a5c000, ioh->op: 0, ioh->state: 1, ioh->result: 0, ioh->num_callbacks: 0, ioh->generation: 42848
!!!pgaio_io_before_start| ioh: 0x105a5c000, ioh->op: 1, ioh->state: 1, ioh->result: 0, ioh->num_callbacks: 2, ioh->generation: 42848

In this run I prohibit writes to io handle in `pgaio_io_acquire_nb` and reenable them in `AsyncReadBuffer`.