osd/PG: store pg_info_t in leveldb (omap), purged_snaps separately

Separate the purged_snaps portion of pg_info_t (the one that gets big).

Add a separate dirty_big_info flag so that we only update the pginfo
"biginfo" file if that state changes.  This lets us avoid the cost in the
general case, like a regular PG write.

Add LEVELDBINFO feature

Put info, biginfo in leveldb

Signed-off-by: Sage Weil <sage@inktank.com>
Signed-off-by: David Zafman <david.zafman@inktank.com>