Scripting: single commands replication mode implemented.

By calling redis.replicate_commands(), the scripting engine of Redis
switches to commands replication instead of replicating whole scripts.
This is useful when the script execution is costly but only results in a
few writes performed to the dataset.

Morover, in this mode, it is possible to call functions with side
effects freely, since the script execution does not need to be
deterministic: anyway we'll capture the outcome from the point of view
of changes to the dataset.

In this mode math.random() returns different sequences at every call.

If redis.replicate_commnads() is not called before any other write, the
command returns false and sticks to whole scripts replication instead.

src/server.c

@@ -1580,6 +1580,7 @@ void initServerConfig(void) {
     server.lpopCommand = lookupCommandByCString("lpop");
     server.rpopCommand = lookupCommandByCString("rpop");
     server.sremCommand = lookupCommandByCString("srem");
+    server.execCommand = lookupCommandByCString("exec");
 
     /* Slow log */
     server.slowlog_log_slower_than = CONFIG_DEFAULT_SLOWLOG_LOG_SLOWER_THAN;