Add bank timestamp bounding (bp #13120) (#13331)

* Add bounding feature

(cherry picked from commit 96b8aa8bd1)

* Repurpose unused as Clock::epoch_start_timestamp; add gated update

(cherry picked from commit 0049ab69fb)

* Add bounded timestamp-estimation method

(cherry picked from commit 80db6c0980)

* Use bounded timestamp-correction when feature enabled

(cherry picked from commit 90778615f6)

* Prevent block times from ever going backward

(cherry picked from commit eb2560e782)

* Sample votes from ancestors back to root

(cherry picked from commit 4260b3b416)

* Add Clock sysvar details, update struct docs

(cherry picked from commit 3a1e125ce3)

* Add design proposal and update validator-timestamp-oracle

(cherry picked from commit a3912bc084)

* Adapt to feature::create_account

Co-authored-by: Tyera Eulberg <tyera@solana.com>
Co-authored-by: Michael Vines <mvines@gmail.com>

sdk/program/src/clock.rs

@@ -79,14 +79,15 @@ pub type UnixTimestamp = i64;
 pub struct Clock {
     /// the current network/bank Slot
     pub slot: Slot,
-    /// unused
-    pub unused: u64,
+    /// the timestamp of the first Slot in this Epoch
+    pub epoch_start_timestamp: UnixTimestamp,
     /// the bank Epoch
     pub epoch: Epoch,
     /// 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!
+    /// originally computed from genesis creation time and network time
+    /// in slots (drifty); corrected using validator timestamp oracle as of
+    /// timestamp_correction and timestamp_bounding features
     pub unix_timestamp: UnixTimestamp,
 }