raft: Avoid sending unnecessary heartbeat when becoming leader.

raft: Avoid sending unnecessary heartbeat when becoming leader.

When a node becomes leader, it sends out heartbeat to all followers
and then sends out another append-request for a no-op command
execution to all followers again immediately. This causes 2
continously append-requests sent out to each followers, and the first
heartbeat append-request is unnecessary. This patch removes the
heartbeat.

Signed-off-by: Han Zhou <hzhou@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>