os/KeyValueDB: Add raw_key() interface for IteratorImpl

raw_key() is useful to split out the prefix.

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>

src/os/KeyValueDB.h

@@ -160,6 +160,9 @@ public:
     string key() {
       return generic_iter->key();
     }
+    pair<string, string> raw_key() {
+      return generic_iter->raw_key();
+    }
     bufferlist value() {
       return generic_iter->value();
     }