osd: limit size of OSDMap cache

osd: limit size of OSDMap cache

If we get way way behind on our maps, we may end up with a really large
OSDMap cache because we currently on trim old maps based on
oldest_last_clean, which may be way in the past.  Avoid eating up gobs of
RAM by putting a ceiling on the cache size.  It'll mean more disk IO in
those situations, but it also means that we'll only load up the old maps
that we actually need (not every single one).

Signed-off-by: Sage Weil <sage.weil@dreamhost.com>