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

util/src/lib.rs

@@ -29,7 +29,7 @@ extern crate lazy_static;
 extern crate serde_derive;
 // Re-export so only has to be included once
 pub use parking_lot::Mutex;
-pub use parking_lot::{RwLock, RwLockReadGuard};
+pub use parking_lot::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 
 // Re-export so only has to be included once
 pub use secp256k1zkp as secp;