Better "final read from parent" algorithm in rewriteAppendOnlyFile*(.
Better "final read from parent" algorithm in rewriteAppendOnlyFile*(.

We now wait up to 1 second for diff data to come from the parent,
however we use poll(2) to wait for more data, and use a counter of
contiguous failures to get data for N times (set to 20 experimentally
after different tests) as an early stop condition to avoid wasting 1
second when the write traffic is too low.

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