osd: make pool_snap_info_t encoding backward compatible

osd: make pool_snap_info_t encoding backward compatible

Way back in fc869dee1e8a1c90c93cb7e678563772fb1c51fb (v0.42) when we redid
the osd type encoding we forgot to make this conditionally encode the old
format for old clients.  In particular, this means that kernel clients
will fail to decode the osdmap if there is a rados pool with a pool-level
snapshot defined.

Fixes: #3290
Signed-off-by: Sage Weil <sage@inktank.com>
Conflicts:

	src/test/encoding/types.h