fix delete parsing for memcache

I think we should support memcache delete argument. because memcached delete spec allow arguments.

See process_delete_command in https://github.com/memcached/memcached/blame/master/memcached.c 
```c
    if (ntokens > 3) {
        bool hold_is_zero = strcmp(tokens[KEY_TOKEN+1].value, "0") == 0;
        bool sets_noreply = set_noreply_maybe(c, tokens, ntokens);
        bool valid = (ntokens == 4 && (hold_is_zero || sets_noreply))
            || (ntokens == 5 && hold_is_zero && sets_noreply);
        if (!valid) {
            out_string(c, "CLIENT_ERROR bad command line format.  "
                       "Usage: delete <key> [noreply]");
            return;
        }
    }
``` 

 so delete command syntax like below:
```c
delete <key>
delete <key> 0
delete <key> <numbers> noreply
``` 

Actually, memcached doesn't handle <numbers> but, it doesn't return ERROR, just it ignores internally.
so. I just follow current memecached behavior. :)