Cache (#126)

* Add middleware/cache

Add a caching middleware that caches nxdomain, nodata and successful
responses. It differentiates between DNSSEC on normal DNS replies.

Each reply is compress and scrubbed so it will fit the specific client
asking for it.

* first simple test, less exporting of stuff

* more

* Add middleware/cache

Add a caching middleware that caches nxdomain, nodata and successful
responses. It differentiates between DNSSEC on normal DNS replies.

Each reply is compressed and scrubbed so it will fit the specific client
asking for it. The TTL is decremented with the time spend in the cache.
There is syntax that allows you to cap the TTL for all records, no
matter what. This allows for a shortlived cache, just to absorb query
peaks.

+Tests

* cache test infrastructure

* Testing

middleware/cache/README.md

@@ -0,0 +1,29 @@
+# cache
+
+`cache` enables a frontend cache.
+
+## Syntax
+
+~~~
+cache [ttl] [zones...]
+~~~
+
+* `ttl` max TTL in seconds, if not specified the TTL of the reply (SOA minimum or minimum TTL in the
+  answer section) will be used.
+* `zones` zones it should should cache for. If empty the zones from the configuration block are used.
+
+
+Each element in the cache is cached according to its TTL, for the negative cache the SOA's MinTTL
+value is used.
+
+A cache mostly makes sense with a middleware that is potentially slow, i.e. a proxy that retrieves
+answer, or to minimize backend queries for middleware like etcd. Using a cache with the file
+middleware essentially doubles the memory load with no concealable increase of query speed.
+
+## Examples
+
+~~~
+cache
+~~~
+
+Enable caching for all zones.