mds: update MClientReconnect encoding

mds: update MClientReconnect encoding

The old encoding assumes that snaprealms are encoded at the tail of
message payload. So it does not allow adding new fields to the message.
The patch introduce new encoding for MClientReconnect, the new encoding
allows us to extend MClientReconnect.

The new encoding is not compatible with the old encoding. If mds does
not understand the new encoding. client needs to use the old encoding
to encode MClientReconnect.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>