ceph: fix pg pool decoding from incremental osdmap update

The incremental map decoding of pg pool updates wasn't skipping
the snaps and removed_snaps vectors.  This caused osd requests
to stall when pool snapshots were created or fs snapshots were
deleted.  Use a common helper for full and incremental map
decoders that decodes pools properly.

Signed-off-by: Sage Weil <sage@newdream.net>