Add directory entries' log compression
Add the dir compact ratio variable
Add the dir log ration setter and getter methods
Keep track of the number of lines in the dir log
Create a method to get the dir cache's log ratio
Create a compact method in RadosFsDir

This calls the respective DirCache's method to compact its
log.
Compact a dir's log if needed, when updated
test: Add unit test for compacting a dir's log
Check if writing a dir's log entry fails