dedupe: allow to generate dedupe buffers from working set

dedupe: allow to generate dedupe buffers from working set

This commit introduced new dedupe generation mode "working_set".
Working set mode simulates a more realistic approach to deduped data,
in which deduped buffers are generated from pre-existing working set -
% size of the device or file.

In other words, dedupe is not usually expected to be close
in time with the source buffer, as well as source buffers
are usually composed of small subset of the entire file or device.

Signed-off-by: Bar David <bardavvid@gmail.com>