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
      *

src/data/CassandraBackend.hpp

@@ -188,11 +188,16 @@ public:
         return {txns, {}};
     }
 
+    void
+    waitForWritesToFinish() override
+    {
+        executor_.sync();
+    }
+
     bool
     doFinishWrites() override
     {
-        // wait for other threads to finish their writes
-        executor_.sync();
+        waitForWritesToFinish();
 
         if (!range_) {
             executor_.writeSync(schema_->updateLedgerRange, ledgerSequence_, false, ledgerSequence_);