mds: send info of imported caps back to the exporter (rename)

use MMDSSlaveRequest::OP_FINISH slave request to send information
of rename imported caps back to the exporter. This is preparation
for including counterpart's information in cap import/export message.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: include counterpart's information in cap import/export messages

when exporting indoes with client caps, the importer sends cap import
messages to clients, the exporter sends cap export messages to clients.
A client can receive these two messages in any order. If a client first
receives cap import message, it adds the imported caps. but the caps
from the exporter are still considered as valid. This can compromise
consistence. If MDS crashes while importing caps, clients can only
receive cap export messages, but don't receive cap import messages.
These clients don't know which MDS is the cap importer, so they can't
send cap reconnect when the MDS recovers.

We can handle above issues by including counterpart's information in
cap import/export messages. If a client first receives cap import
message, it added the imported caps, then removes the the exporter's
caps. If a client first receives cap export message, it removes the
exported caps, then adds caps for the importer.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: send cap import messages to clients after importing subtree success

When importing subtree, the importer sends cap import messages to clients
before the import subtree operation is considered as success. If the
exporter crashes before EExport event is journalled, the importer needs to
re-export client caps. This confuses clients, and makes them lose track of
auth caps.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
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 exporter's caps (send release to the exporter).
This saves the cap export message and a "mds to mds" 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>
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 cap grant message that updates max size.

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