osd: drop default map cache size from 500 -> 200

osd: drop default map cache size from 500 -> 200

OSDs don't need this from each other anymore since they send
a pg_interval_map_t with pg notify.  This cache is only used
during startup when loading things up (and 200 is still > the
max advance of 150), and when clients appear with super old maps
(pretty unusual unless they are idle for long periods).

It should mean a pretty significant cut in memory...

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