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

programs/vote/src/vote_instruction.rs

@@ -33,6 +33,9 @@ pub enum VoteError {
 
     #[error("vote has no slots, invalid")]
     EmptySlots,
+
+    #[error("vote timestamp not recent")]
+    TimestampTooOld,
 }
 impl<E> DecodeError<E> for VoteError {
     fn type_of() -> &'static str {