add sync() to db abstraction for fsync(2).

src/database/abstraction.rs

@@ -18,6 +18,7 @@ fn open(config: &Config) -> Result<Self>
 		Self: Sized;
 	fn open_tree(&self, name: &'static str) -> Result<Arc<dyn KvTree>>;
 	fn flush(&self) -> Result<()>;
+	fn sync(&self) -> Result<()> { Ok(()) }
 	fn cleanup(&self) -> Result<()> { Ok(()) }
 	fn memory_usage(&self) -> Result<String> {
 		Ok("Current database engine does not support memory usage reporting.".to_owned())

src/database/abstraction/rocksdb.rs

@@ -170,12 +170,17 @@ fn open_tree(&self, name: &'static str) -> Result<Arc<dyn KvTree>> {
 	}
 
 	fn flush(&self) -> Result<()> {
-		debug!("Running flush_wal (no sync)");
 		rust_rocksdb::DBCommon::flush_wal(&self.rocks, false)?;
 
 		Ok(())
 	}
 
+	fn sync(&self) -> Result<()> {
+		rust_rocksdb::DBCommon::flush_wal(&self.rocks, true)?;
+
+		Ok(())
+	}
+
 	fn memory_usage(&self) -> Result<String> {
 		let stats = rust_rocksdb::perf::get_memory_usage_stats(Some(&[&self.rocks]), Some(&[&self.cache]))?;
 		Ok(format!(