Add cache_capacity option to dnssec middleware for the capacity of LRU cache (#339)

This fix adds a `cache_capacity` option to dnssec middleware, so that
it is possible to specify the capacity of the LRU cache used by dnssec
middleware.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

middleware/dnssec/README.md

@@ -26,6 +26,7 @@ TODO(miek): think about key rollovers, and how to do them automatically.
 ~~~
 dnssec [ZONES... ] {
     key file KEY...
+    cache_capacity CAPACITY
 }
 ~~~
 
@@ -33,4 +34,9 @@ dnssec [ZONES... ] {
   will be signed with all keys. Generating a key can be done with `dnssec-keygen`: `dnssec-keygen -a
   ECDSAP256SHA256 <zonename>`. A key created for zone *A* can be safely used for zone *B*.
 
+
+* `cache_capacity` indicates the capacity of the LRU cache. The dnssec middleware uses LRU cache to manage
+  objects and the default capacity is 10000.
+
+
 ## Examples