Support touch command

[jaderhs]

The "touch" command is used to update the expiration time of an existing item
without fetching it.

touch [noreply]\r\n

Would be an interesting feature.

[agentzh]

This support is implemented in the lua-resty-memcached library:

https://github.com/agentzh/lua-resty-memcached

You can use it for now.

I don't mind adding support for "touch" to ngx_memc but I don't have the time to implement it myself. Patches welcome :)