PSYNC2: Save Lua scripts state into RDB file.

This is currently needed in order to fix #4483, but this can be
useful in other contexts, so maybe later we may want to remove the
conditionals and always save/load scripts.

Note that we are using the "lua" AUX field here, in order to guarantee
backward compatibility of the RDB file. The unknown AUX fields must be
discarded by past versions of Redis.

src/server.h

@@ -1781,6 +1781,7 @@ void scriptingInit(int setup);
 int ldbRemoveChild(pid_t pid);
 void ldbKillForkedSessions(void);
 int ldbPendingChildren(void);
+int luaCreateFunction(client *c, lua_State *lua, char *funcname, robj *body);
 
 /* Blocked clients */
 void processUnblockedClients(void);