Remove some optimization and lowercasing of the qname (in the end
miekg/dns should provide a fast and OK function for it).

* remove the make([]byte, 2) allocation in the key()
* use already lowercased qname in hash key calculation.

% benchcmp old.txt new.txt
benchmark                    old ns/op     new ns/op     delta
BenchmarkCacheResponse-4     9599          8735          -9.00%
Signed-off-by: Miek Gieben <miek@miek.nl>