Add intermittent Timestamping to Votes (#7233)

* Add intermittent timestamp to Vote

* Add timestamp to VoteState, add timestamp processing to program

* Print recent timestamp with solana show-vote-account

* Add offset of 1 to timestamp Vote interval to initialize at node boot (slot 1)

* Review comments

* Cache last_timestamp in Tower and use for interval check

* Move work into Tower method

* Clarify timestamping interval

* Replace tuple with struct

cli/src/vote.rs

@@ -353,6 +353,7 @@ pub fn process_show_vote_account(
             None => "~".to_string(),
         }
     );
+    println!("recent timestamp: {:?}", vote_state.last_timestamp);
     if !vote_state.votes.is_empty() {
         println!("recent votes:");
         for vote in &vote_state.votes {