pool: fix lost space messages (and cause)

Since version 5.0.84 Berkeley DB generates statistics files like je.stat.csv,
which make pool space calculation unhappy. As we don't use this
information, disable to avoid annoying messages about lost space
(even in case of read-only activities):

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26 Feb 2014 16:36:55 (dcache-lab002-A) [] The file system containing the data files appears to
have less free space (6,094,454,210,560 bytes) than expected (6,094,454,214,656 bytes);
reducing the pool size to 16,435,147,381,007 bytes to compensate. Notice that this does not
leave any space for the meta data. If such data is stored on the same file system, then it is paramount
that the pool size is reconfigured to leave enough space for the meta data.
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Acked-by: Gerd Behrmann
Target: master
Require-book: no
Require-notes: no