New API to force propagation.

The old REDIS_CMD_FORCE_REPLICATION flag was removed from the
implementation of Redis, now there is a new API to force specific
executions of a command to be propagated to AOF / Replication link:

    void forceCommandPropagation(int flags);

The new API is also compatible with Lua scripting, so a script that will
execute commands that are forced to be propagated, will also be
propagated itself accordingly even if no change to data is operated.

As a side effect, this new design fixes the issue with scripts not able
to propagate PUBLISH to slaves (issue #873).

src/pubsub.c

@@ -306,6 +306,7 @@ void punsubscribeCommand(redisClient *c) {
 
 void publishCommand(redisClient *c) {
     int receivers = pubsubPublishMessage(c->argv[1],c->argv[2]);
+    forceCommandPropagation(c,REDIS_PROPAGATE_REPL);
     addReplyLongLong(c,receivers);
 }