Set server.repl_down_since to 0 when changing master.

When an instance is potentially set to replicate with another master, it
is conceptually disconnected forever, since we have no old copy of the
dataset for this master in memory.

src/replication.c

@@ -1261,6 +1261,7 @@ void replicationSetMaster(char *ip, int port) {
     cancelReplicationHandshake();
     server.repl_state = REDIS_REPL_CONNECT;
     server.master_repl_offset = 0;
+    server.repl_down_since = 0;
 }
 
 /* Cancel replication, setting the instance as a master itself. */