fix: Reduce the previous block waiting time

Reduce the amount of time that the block verifier waits for the previous
block, before polling again.

(Waiting for 2 seconds resulted in some apparent block verifier hangs.)

This is a temporary fix, until the state layer handles context checks.

zebra-consensus/src/block.rs

@@ -185,7 +185,7 @@ where
                 //   - Replace with AddBlock constraints
                 None => {
                     tracing::debug!(?height, ?hash, "Waiting for state to have block");
-                    time::delay_for(Duration::from_secs(2)).await
+                    time::delay_for(Duration::from_millis(50)).await
                 }
             };
         }