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The scratch_files feature is not really needed except on 32-bit
platforms, since tdb's performance is pretty awful given how we are
using it.  Maybe SQLite would be faster, but for 64-bit platforms,
enabling swap works fairly well, especially using the rbtree for the
bitmap abstraction.

We leave tdb for Android since it's unlikely that someone will be
trying to connect petabyte+ sized file systems to a mobile handset.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>