OSD: only trim up to the oldest map still in use by a pg

map_cache.cached_lb() provides us with a lower bound across
all pgs for in-use osdmaps.  We cannot trim past this since
those maps are still in use.

backport: bobtail
Fixes: #3770
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>