Correct Bank timestamp drift every slot (#12737)

* Move timestamp helper to sdk

* Add Bank method for getting timestamp estimate

* Return sysvar info from Bank::clock

* Add feature-gated timestamp correction

* Rename unix_timestamp method to be more descriptive

* Review comments

* Add timestamp metric

sdk/src/lib.rs

@@ -45,6 +45,7 @@ pub mod short_vec;
 pub mod slot_hashes;
 pub mod slot_history;
 pub mod stake_history;
+pub mod stake_weighted_timestamp;
 pub mod system_instruction;
 pub mod system_program;
 pub mod sysvar;