using sync mutex to prevent race condition
eg : imagine two requests for the same url example.com via 2 goroutines , assume count of url =0;
first goroutine will increment the counter by 1 ; updated count =1; second goroutine will also increment the counter by 1 ; updated count=1; without mutext , we will get udpdated count=1; because the second goroutine will also read the same initial value of count=0;
defer statements are eecuted when the function returns ;
