Add schema::set_comment() member function

Needed by the cf_prop_defs class to modify a schema object in the
apply_to_cf_metadata() function.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

schema.hh

@@ -114,6 +114,9 @@ public:
         shared_ptr<abstract_type> regular_column_name_type,
         sstring comment = {});
     schema(const schema&);
+    void set_comment(const sstring& comment) {
+        _comment = comment;
+    }
     bool is_dense() const {
         return false;
     }