fseek() in File::append() was invalidating the file buffer for each call
to append(). Better handling improves "import" performance test by 9%,
45% in "commit", on a system with a spinning disk.
This performance problem affects all operations where
{pending,completed}.data are rewritten. During normal operation a
garbage collection can be enough to trigger it. On storage with high
latency, e.g. networked, this previously took 20 seconds and more.
