Hook up getBlockTime rpc to real data (#7476)

* Add blocktree timestamp helper functions and tests

* Flesh out blocktree::get_block_time

* Move stakes up into rpc to make testing easier; expand tests

* Review comments

programs/vote/src/vote_state.rs

@@ -26,7 +26,7 @@ pub const INITIAL_LOCKOUT: usize = 2;
 //  smaller numbers makes
 pub const MAX_EPOCH_CREDITS_HISTORY: usize = 64;
 
-// Frequency of timestamp Votes In v0.22.0, this is approximately 30min with cluster clock
+// Frequency of timestamp Votes. In v0.22.0, this is approximately 30min with cluster clock
 // defaults, intended to limit block time drift to < 1hr
 pub const TIMESTAMP_SLOT_INTERVAL: u64 = 4500;