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/errors.rs

@@ -48,6 +48,7 @@ pub enum BeaconChainError {
     /// Returned when an internal check fails, indicating corrupt data.
     InvariantViolated(String),
     SszTypesError(SszTypesError),
+    CanonicalHeadLockTimeout,
 }
 
 easy_from_to!(SlotProcessingError, BeaconChainError);