mds: flush session messages before exporting caps

Following sequence of events can happen when exporting inodes:

- client sends open file request to mds.0
- mds.0 handles the request and sends inode stat back to the client
- mds.0 export the inode to mds.1
- mds.1 sends cap import message to the client
- mds.0 sends cap export message to the client
- client receives the cap import message from mds.1, but the client
  still doesn't have corresponding inode in the cache. So the client
  releases the imported caps.
- client receives the open file reply from mds.0
- client receives the cap export message from mds.0.

After the end of these events, the client doesn't have any cap for
the opened file.

To fix the message ordering issue, this patch introduces a new session
operation FLUSHMSG. Before exporting caps, we send a FLUSHMSG seesion
message to client and wait for the acknowledgment. When receiveing the
FLUSHMSG_ACK message from client, we are sure that clients have received
all messages sent previously.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: send info of imported caps back to the exporter (export dir)

Introduce a new class Capability::Import and use it to send information
of 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: send info of imported caps back to the exporter (cache rejoin)

Use cache rejoin ack message to send information of rejoin imported
caps back to the exporter. Also move the code that exports reconnect
caps to MDCache::handle_cache_rejoin_ack()

This is preparation for including counterpart's information in cap
import/export message.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
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: re-send cap exports in resolve message.

For rename operation that changes inode's authority, if master mds
of the operation crashed, inode's original auth mds sends export
messages to clients when it receives the master mds' resolve ack
message, Client can't reply on the export message to add caps for
the master mds, then reconnect the cap when the master mds enters
reconnect stage. Because client may receive the export message after
receiving mdsmap that claims the master mds is in reconnect stage.

The fix is include cap exports in resolve message, so the master mds
can send import messages to clients when it enters the rejoin stage.

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

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>
client: handle session flush 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>