consensus, state: increase Buffer sizes

Using a Buffer with size 1 is a footgun because it allows only one
sender to call poll_ready at a time.  This is usually undesirable
because it means that a task or service that calls poll_ready but only
makes a service call later (potentially much later) will block all other
callers.

zebra-consensus/src/chain.rs

@@ -152,6 +152,6 @@ where
             max_checkpoint_height,
             last_block_height: None,
         }),
-        1,
+        3,
     )
 }

zebra-state/src/service.rs

@@ -104,5 +104,5 @@ pub fn init(
     config: Config,
     network: Network,
 ) -> Buffer<BoxService<Request, Response, BoxError>, Request> {
-    Buffer::new(BoxService::new(StateService::new(config, network)), 1)
+    Buffer::new(BoxService::new(StateService::new(config, network)), 3)
 }