ZPOP: change sync ZPOP to have a count argument instead of N keys.

Usually blocking operations make a lot of sense with multiple keys so
that we can listen to multiple queues (or whatever the app models) with
a single connection. However in the synchronous case it is more useful
to be able to ask for N elements. This is a change that I also wanted to
perform soon or later in the blocking list variant, but here it is more
natural since there is no reply type difference.

src/blocked.c

@@ -331,7 +331,7 @@ void handleClientsBlockedOnKeys(void) {
                                      receiver->lastcmd->proc == bzpopminCommand)
                                      ? ZSET_MIN : ZSET_MAX;
                         unblockClient(receiver);
-                        genericZpopCommand(receiver,&rl->key,1,where);
+                        genericZpopCommand(receiver,&rl->key,1,where,1,NULL);
 
                         propagate(where == ZSET_MIN ?
                             server.zpopminCommand : server.zpopmaxCommand,