Backport: add first_block_num for cleos get_info

[linh2931]

EOSIO/eos#10087

[matthewdarwin]

This is done already?

[matthewdarwin]

see "earliest_available_block_num" in screenshot.

[linh2931]

Thanks @matthewdarwin. Checked the code in Mandel and EOSIO. They refer to the same.

In Mandel, controller.cpp has

   uint32_t earliest_available_block_num() const {
      return (blog.first_block_num() != 0) ? blog.first_block_num() : fork_db.root()->block_num;
   }

In EOSIO 2.2, controller.cpp has

uint32_t controller::get_first_block_num()const
{
   return my->blog.get_first_block_num();
}

Will close this.