[PATCH] Fixed and polished SMP store stats collection

SMP shared memory cache stats were not collected.

Mean disk object size stats were aggregated inaccurately for SMP.

Moved Store-related stats into a dedicated StoreStats class,
encapsulating memory cache-related (mem), disk cache-related (swap), and
global store (number of objects) stats. Used consistent naming scheme to
make memory and disk stats more alike (we could create a class to
represent "store device" stats of sorts, but that seems like an overkill
at this time).

Moved Store stats collection into corresponding Store classes rather
than forcing GetInfo() in stat.cc to know how to deal with all Store stats.

The new code seems to work in my very limited tests but I am pretty sure
more stats/SMP-related fixes will be needed.

I would also like to polish mgr:info display of Store stats along the
same mem/disk symmetry lines, remove duplicated, and add missing
information there, but I am worried that changing mgr:info output format
will break old scripts. Perhaps we should add a new mgr:store action
instead?

Thank you,

Alex.