fix problem with finding most recent irreversible block after hived restart
Sometimes happens when found irreversible block is lesser than the current one. The issue is hard to reproduce and is connected with inserting fork event (which is also inserted when hived restarts after a break) during executing 'hive.app_next_block'
additionally, the logic of updates of a context's irreversible block is reduced only to hive.update_irreversible function
Edited by Marcin