[thin_check (rust)] Move mutex inside IoEngines.

Makes it easier to share an engine between different threads.

src/thin/check.rs

@@ -10,7 +10,7 @@ use std::thread::{self, spawn};
 use std::time::{Duration, Instant};
 use threadpool::ThreadPool;
 
-use crate::block_manager::{AsyncIoEngine, Block, IoEngine, SyncIoEngine, BLOCK_SIZE};
+use crate::block_manager::{AsyncIoEngine, Block, IoEngine, BLOCK_SIZE};
 use crate::checksum;
 use crate::thin::superblock::*;