Accountsdb replication installment 2 (#19325)

This is the 2nd installment for the AccountsDb replication.

Summary of Changes

The basic google protocol buffer protocol for replicating updated slots and accounts. tonic/tokio is used for transporting the messages.

The basic framework of the client and server for replicating slots and accounts -- the persisting of accounts in the replica-side will be done at the next PR -- right now -- the accounts are streamed to the replica-node and dumped. Replication for information about Bank is also not done in this PR -- to be addressed in the next PR to limit the change size.

Functionality used by both the client and server side are encapsulated in the replica-lib crate.

There is no impact to the existing validator by default.

Tests:

Observe the confirmed slots replicated to the replica-node.
Observe the accounts for the confirmed slot are received at the replica-node side.

runtime/src/accounts_cache.rs

@@ -134,6 +134,9 @@ impl CachedAccountInner {
             }
         }
     }
+    pub fn pubkey(&self) -> Pubkey {
+        self.pubkey
+    }
 }
 
 #[derive(Debug, Default)]