Refactor SyncState (#3297)

* Refactor SyncState

Method sync_error() retrun type was simplified.
update_txhashset_download() was  made type safe, which eliminates a runtime enum variant's  check, added an atomic status update

api/src/handlers/server_api.rs

@@ -79,16 +79,11 @@ fn sync_status_to_api(sync_status: SyncStatus) -> (String, Option<serde_json::Va
 			"header_sync".to_string(),
 			Some(json!({ "current_height": current_height, "highest_height": highest_height })),
 		),
-		SyncStatus::TxHashsetDownload {
-			start_time: _,
-			prev_update_time: _,
-			update_time: _,
-			prev_downloaded_size: _,
-			downloaded_size,
-			total_size,
-		} => (
+		SyncStatus::TxHashsetDownload(stats) => (
 			"txhashset_download".to_string(),
-			Some(json!({ "downloaded_size": downloaded_size, "total_size": total_size })),
+			Some(
+				json!({ "downloaded_size": stats.downloaded_size, "total_size": stats.total_size }),
+			),
 		),
 		SyncStatus::TxHashsetRangeProofsValidation {
 			rproofs,