makes crds fields private (#13703)

Crds fields should maintain several invariants between themselves, so
exposing them as public fields can be bug prone. In addition these
invariants are asserted on every write:
https://github.com/solana-labs/solana/blob/9668dd85d/core/src/crds.rs#L138-L154
https://github.com/solana-labs/solana/blob/9668dd85d/core/src/crds.rs#L239-L262
which adds extra instructions and is not optimal. Should these fields be
private the asserts will be redundant.

core/tests/crds_gossip.rs

@@ -498,7 +498,7 @@ fn network_run_pull(
         }
         let total: usize = network_values
             .par_iter()
-            .map(|v| v.lock().unwrap().crds.table.len())
+            .map(|v| v.lock().unwrap().crds.len())
             .sum();
         convergance = total as f64 / ((num * num) as f64);
         if convergance > max_convergance {