Add timeouts to canonical head rwlock (#759)

* Add TimeoutRwLock to BeaconChain

* Update network crate

* Update rest api

* Fix beacon chain tests

* Fix rest api tests

* Set test back to !debug_assertions

beacon_node/beacon_chain/src/timeout_rw_lock.rs

@@ -0,0 +1,20 @@
+use parking_lot::{RwLock, RwLockReadGuard, RwLockWriteGuard};
+use std::time::Duration;
+
+/// A simple wrapper around `parking_lot::RwLock` that only permits read/write access with a
+/// time-out (i.e., no indefinitely-blocking operations).
+pub struct TimeoutRwLock<T>(RwLock<T>);
+
+impl<T> TimeoutRwLock<T> {
+    pub fn new(inner: T) -> Self {
+        Self(RwLock::new(inner))
+    }
+
+    pub fn try_read_for(&self, timeout: Duration) -> Option<RwLockReadGuard<T>> {
+        self.0.try_read_for(timeout)
+    }
+
+    pub fn try_write_for(&self, timeout: Duration) -> Option<RwLockWriteGuard<T>> {
+        self.0.try_write_for(timeout)
+    }
+}