Don't update database-level statistics on every page cache operation.

[hvlad]

Statistics counters at database-level is shared by all attachments in SS architecture.
Operations in page cache (in particular fetches) is most frequent operations that changes corresponding counters.
When concurrent attachments actively works with page cache and often increments corresponding counters it leads
to the bouncing of memory area where counters allocated between CPU cores, and creates performance penalty.

The improvement is to update database-level stats only when necessary making less changes in common memory.

[hvlad]

Re-opened to fix failure of QA test for CORE-3625 (#3977)