Get block by time stamp (#3514)

* initial

* binary search through blocks

* snapshot

* returning highest block

* oldest block is now 0

* multiple test written for  getBlockByTimeStamp

* retrieved picked block time stamp

* added into erigon_api

* returning pointer

* reverting c37b99b2

* deleted print

* proper rpc block response

* fixing test

* returning lowest block

* lint

* typo

* fixed test

* returning lowest-1

* skip while loop for times too big or small

* added to readme

* using sort.Search