md-cluster: fix locking when node joins cluster during message broadcast
If a node joins the cluster while a message broadcast
is under way, a lock issue could happen as follows.
For a cluster which included two nodes, if node A is
calling __sendmsg before up-convert CR to EX on ack,
and node B released CR on ack. But if a new node C
joins the cluster and it doesn't receive the message
which A sent before, so it could hold CR on ack before
A up-convert CR to EX on ack.
So a node joining the cluster should get an EX lock on
the "token" first to ensure no broadcast is ongoing,
then release it after held CR on ack.