Cluster: call clusterHandleSlaveFailover() when our master is down.
Replication: master_link_down_since_seconds initial value should be huge.

server.repl_down_since used to be initialized to the current time at
startup. This is wrong since the replication never started. Clients
testing this filed to check if data is uptodate should never believe
data is recent if we never ever connected to our master.
Cluster: clusterHandleSlaveFailover() stub.