feat: Graceful shutdown (#1801)

Fixes #442.

src/data/BackendInterface.hpp

@@ -683,6 +683,12 @@ public:
     bool
     finishWrites(std::uint32_t ledgerSequence);
 
+    /**
+     * @brief Wait for all pending writes to finish.
+     */
+    virtual void
+    waitForWritesToFinish() = 0;
+
     /**
      * @brief Mark the migration status of a migrator as Migrated in the database
      *