osd: need feature bit for v0.42 osdmap encoding change
osd: conditionally encode old pg_pool_t when no CEPH_FEATURE_OSDENC
This fixes OSDMap compatibility between v0.42 and <v0.42.
For MOSDMap, reencode maps if OSDENC feature is missing. Also rev the
message version. We don't use COMPAT version here because v3 can't be
understood by v2 (that's why we're checking feature bits). (It will be
possible to do that later when our constituent types can be decoded by