Use the first block in an era as the basis for counting era length

[SaiProServ]

Currently eras end after a specific interval has passed since the era start time, which is based on the timestamp of the switch block from the previous era. This causes lower rewards when some initial blocks are skipped due to long processing of the switch block by the validators. In order to fix this, we should count era length from the timestamp of the first proposed block in the era instead.

[cspramit]

Which release @SaiProServ