mds: simplify how to export non-auth caps

Introduce a new flag in cap import message. If client finds the flag
is set, it releases the exporter's caps (send release to the exporter).
This saves the cap export message and a "importer to exporter" message.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: add CEPH_FEATURE_EXPORT_PEER and bump the protocal version

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