Test: check that Redis starts empty without an RDB file.
Test: more RDB loading checks.

A test for issue #1001 is included.
Test: make sure broken RDB checksum is detected.
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.
Cluster: FAILOVER_AUTH_REQUEST message type introduced.

This message is sent by a slave that is ready to failover its master to
other nodes to get the authorization from the majority of masters.
Cluster: use 'else if' for mutually exclusive conditionals.
Cluster: sanity check FAILOVER_AUTH_REQUEST messages for proper length.
Cluster: handle FAILOVER_AUTH_REQUEST in clusterProcessPacket().

However currently the control is passed to a function doing nothing at
all.
Cluster: clusterSendFailoverAuthIfNeeded() work in progress.