Implement Close for prepared statements (#482)

* Partial support for Close

* Close

* respect config value

* prepared spec

* Hmm

* Print cache size

src/admin.rs

@@ -701,6 +701,7 @@ where
         ("age_seconds", DataType::Numeric),
         ("prepare_cache_hit", DataType::Numeric),
         ("prepare_cache_miss", DataType::Numeric),
+        ("prepare_cache_size", DataType::Numeric),
     ];
 
     let new_map = get_server_stats();
@@ -732,6 +733,10 @@ where
                 .prepared_miss_count
                 .load(Ordering::Relaxed)
                 .to_string(),
+            server
+                .prepared_cache_size
+                .load(Ordering::Relaxed)
+                .to_string(),
         ];
 
         res.put(data_row(&row));