Explicit allow to enable / disable session cache

Motivation:

It is sometimes useful to enable / disable the session cache.

Modifications:

* Add OpenSslSessionContext.setSessionCacheEnabled(...) and isSessionCacheEnabled()

Result:

It is now possible to enable / disable cache on the fly

handler/src/main/java/io/netty/handler/ssl/OpenSslClientContext.java

@@ -214,5 +214,15 @@ public final class OpenSslClientContext extends OpenSslContext {
         public int getSessionCacheSize() {
             return 0;
         }
+
+        @Override
+        public void setSessionCacheEnabled(boolean enabled) {
+            // ignored
+        }
+
+        @Override
+        public boolean isSessionCacheEnabled() {
+            return false;
+        }
     }
 }

handler/src/main/java/io/netty/handler/ssl/OpenSslServerContext.java

@@ -319,5 +319,16 @@ public final class OpenSslServerContext extends OpenSslContext {
         public int getSessionCacheSize() {
             return (int) SSLContext.getSessionCacheSize(context);
         }
+
+        @Override
+        public void setSessionCacheEnabled(boolean enabled) {
+            long mode = enabled ? SSL.SSL_SESS_CACHE_SERVER : SSL.SSL_SESS_CACHE_OFF;
+            SSLContext.setSessionCacheMode(context, mode);
+        }
+
+        @Override
+        public boolean isSessionCacheEnabled() {
+            return SSLContext.getSessionCacheMode(context) == SSL.SSL_SESS_CACHE_SERVER;
+        }
     }
 }

handler/src/main/java/io/netty/handler/ssl/OpenSslSessionContext.java

@@ -59,6 +59,16 @@ public abstract class OpenSslSessionContext implements SSLSessionContext {
         SSLContext.setSessionTicketKeys(context, keys);
     }
 
+    /**
+     * Enable or disable caching of SSL sessions.
+     */
+    public abstract void setSessionCacheEnabled(boolean enabled);
+
+    /**
+     * Return {@code true} if caching of SSL sessions is enabled, {@code false} otherwise.
+     */
+    public abstract boolean isSessionCacheEnabled();
+
     /**
      * Returns the stats of this context.
      */