storage_service: incremental backups

Query and set the state of incremental backups. The initial value comes from
the configuration file through the local db reference. Later on, it can be
changed through the interface.

Signed-off-by: Glauber Costa <glommer@scylladb.com>

service/storage_service.hh

@@ -85,6 +85,14 @@ public:
     }
     static int RING_DELAY; // delay after which we assume ring has stablized
 
+    bool incremental_backups_enabled() {
+        return _cfg->incremental_backups();
+    }
+
+    void incremental_backups_set_value(bool val) {
+        _cfg->incremental_backups() = val;
+    }
+
     // Needed by distributed<>
     future<> stop();