mds: add CEPH_FEATURE_EXPORT_PEER and bump the protocal version

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: increase cap sequence when sharing max size

For case:
 - client voluntarily releases some caps through cap update message
 - mds shares the new max by sending cap grant message
 - mds recevies the cap update message

If mds doesn't increase the cap sequence when sharing the max size.
It can't determine if the cap update message was sent before or after
client reveived the max size.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>