crimson/os/seastore: measure transactional efforts that are discarded or committed

crimson/os/seastore: measure transactional efforts that are discarded or committed

The efforts of a transaction include the number and bytes of its read,
mutate, retire and fresh extents, and the bytes of delta generated.

This helps to understand the following aspects:
* The ratio of discarded efforts vs committed efforts;
* The average efforts of a transaction;
* The distribution of read/mutate/delta/retire/fresh efforts;
* The memory overhead and potential disk overhead of a transaction;
* How early a transaction invalidation happens;
* The average extent length;

It is possible to extend the effort metrics to be labeled by extent
types, in case we want to distinguish and profile the efforts at the
sub-component level.

Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>