Add time since genesis to sysvar::clock (#7289)

* genesis timestamp

* remove clock::create_account()

* ..

* add configure-able creation time

* dividing by 1T, should be dividing by 1B

sdk/src/clock.rs

@@ -108,6 +108,9 @@ pub struct Clock {
     /// the future Epoch for which the leader schedule has
     ///  most recently been calculated
     pub leader_schedule_epoch: Epoch,
+    /// computed from genesis creation time and network time
+    ///  in slots, drifts!
+    pub unix_timestamp: UnixTimestamp,
 }
 
 #[cfg(test)]