threadpool: add panic_handler (#1052)

tokio-threadpool/src/config.rs

@@ -1,5 +1,6 @@
 use callback::Callback;
 
+use std::any::Any;
 use std::fmt;
 use std::sync::Arc;
 use std::time::Duration;
@@ -14,6 +15,7 @@ pub(crate) struct Config {
     pub around_worker: Option<Callback>,
     pub after_start: Option<Arc<Fn() + Send + Sync>>,
     pub before_stop: Option<Arc<Fn() + Send + Sync>>,
+    pub panic_handler: Option<Arc<Fn(Box<Any + Send>) + Send + Sync>>,
 }
 
 /// Max number of workers that can be part of a pool. This is the most that can