client: print out mode, uid, gid if they are changed

These values can be printed by MClientRequest message output, but if
they're flushed out via cap updates we don't even get that. These
are important values that generally don't change often; so print
them out in debug logging.

Signed-off-by: Greg Farnum <greg@inktank.com>
Merge pull request #2676 from ceph/wip-print-mode

client: print out mode, uid, gid if they are changed