Separated touch and delete

Apparently, the delete command does not really support a time parameter
even though the code had support for it -
https://github.com/memcached/memcached/blob/master/doc/protocol.txt#L254
Also, found that the expiry parameter is mandatory in touch -
https://github.com/memcached/memcached/blob/master/doc/protocol.txt#L324.
For these reasons, it makes more sense to use separate functions.