Remove the expire test

This clashes with the cache testing/benchmarking and more importantly
it actually tests an etcd feature (=correctly expiring stuff).
Implement and finish response caching

The benchmark tests run x5 faster:

PASS
BenchmarkDNSSingle     10000        187069 ns/op
BenchmarkDNSWildcard   10000        130846 ns/op
BenchmarkDNSSECSingle  10000        258639 ns/op

Coming from:

PASS
BenchmarkDNSSingle      2000        780097 ns/op
BenchmarkDNSWildcard    2000       1031535 ns/op
BenchmarkDNSSECSingle   5000        869941 ns/op
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