osd, PG: share the snaptrimq.size() within pg_stat_t

osd, PG: share the snaptrimq.size() within pg_stat_t

That way it will be unnecessary to go through all pgs separately
to find pgs with excessively long snap trim queues. And we don't need
to share snap trim queues itself, which may be large by itself.
As snap trim queues tend to be short and anything above 50 000
I consider absurdly large, the snaptrimq_len is capped at 2^32 to
save space in pg_stat_t.

Signed-off-by: Piotr Dałek <piotr.dalek@corp.ovh.com>