add backoff for perssimistic

Signed-off-by: haojinming <jinming.hao@pingcap.com>

Author: haojinming

src/backoff.rs

@@ -7,7 +7,7 @@ use std::time::Duration;
 
 pub const DEFAULT_REGION_BACKOFF: Backoff = Backoff::no_jitter_backoff(2, 500, 10);
 pub const OPTIMISTIC_BACKOFF: Backoff = Backoff::no_jitter_backoff(2, 500, 10);
-pub const PESSIMISTIC_BACKOFF: Backoff = Backoff::no_backoff();
+pub const PESSIMISTIC_BACKOFF: Backoff = Backoff::no_jitter_backoff(2, 500, 10);
 
 /// When a request is retried, we can backoff for some time to avoid saturating the network.
 ///