Rename ValidatorExit and move to sdk (#17728)

sdk/src/exit.rs

@@ -0,0 +1,30 @@
+use std::fmt;
+
+#[derive(Default)]
+pub struct Exit {
+    exited: bool,
+    exits: Vec<Box<dyn FnOnce() + Send + Sync>>,
+}
+
+impl Exit {
+    pub fn register_exit(&mut self, exit: Box<dyn FnOnce() + Send + Sync>) {
+        if self.exited {
+            exit();
+        } else {
+            self.exits.push(exit);
+        }
+    }
+
+    pub fn exit(&mut self) {
+        self.exited = true;
+        for exit in self.exits.drain(..) {
+            exit();
+        }
+    }
+}
+
+impl fmt::Debug for Exit {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{} exits", self.exits.len())
+    }
+}