Add touch to the ASCII protocol.

Touch is defined for the ASCII memcached protocol, but not implemented in
spymemcached. This commit adds the missing touch functionality.

This changeset also modifies the binary touch to make it work even when
assertions are enabled.

Change-Id: I4227441cf42f4e75b770952aa8c8f8e5efa30e66
Reviewed-on: http://review.couchbase.org/26513
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>