os/bluestore/compression: Estimator class

os/bluestore/compression: Estimator class

Add CMake rules to compile.
Add bluestore_compression dout subsys.

Created Estimator class.
It is used by Scanner to decide if specific extent is to be recompressed.
Prepare for future machine learning / adaptive algorithm for estimation.

So far logic of Estimator is relatively simple.
It learns expected recompression values and uses them in next iterations to predict.

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>