Timestamp weights (#5775)

* calculate weight for timestamp::set

* reduce db count in kill_identity weight calculation

* return weight of on_finalize in on_initialize

* add comment

* import Weight

* address review comments to update db weight count

* fix full block import test

* update weights and benchmark info to latest benchmark data

* update identity pallet weights and benchmark info

Co-authored-by: Shawn Tabrizi <[email protected]>