miner/oracle: use activeset from reference ballot

[dshulyak]

if node created reference ballot with graded activeset and later was restarted it will create an activeset from first block.
such ballots will be syntactically invalid as receiver will validate them according to the reference ballot, that used graded active set.

this needs to be fixed before next epoch begins, otherwise network will very likely die

the solution is to lookup activeset on a reference ballot that stored locally, there is already code for it.