Avoid verbose default logs

Temporary fix so that Zebra's default logs support a typical workflow:
1. Developer or user runs Zebra with the default config
2. They send the logs to a terminal
3. When they see a bug, they copy-paste the last few log lines into a
   bug report

This is the same change that was merged in #1373 and reverted in #1375.
We'll create a consistent logging design for Zebra in ticket #1381.

zebra-state/src/service/non_finalized_state/chain.rs

@@ -33,7 +33,7 @@ impl Chain {
     pub fn push(&mut self, block: PreparedBlock) {
         // update cumulative data members
         self.update_chain_state_with(&block);
-        tracing::info!(block = %block.block, "adding block to chain");
+        tracing::debug!(block = %block.block, "adding block to chain");
         self.blocks.insert(block.height, block);
     }