Basic support for dedupe

This adds and option, dedupe_percentage, that controls how many of
the write IO buffers are identical. For instance, if this is set:

dedupe_percentage=70

then 70% of the write IO buffers will have identical contents. The
specific contents are, as before, controlled by the various options
that set buffer contents or buffer compressibility.

Signed-off-by: Jens Axboe <axboe@fb.com>