Use blockHash for AcceptedWorkID, not powHash.

blockHash is used to construct the AcceptedWorkID when retrieving
AcceptedWork from the database, not the powHash.

Because of this bug, dcrpool was failing to register when a block mined
by the pool had been confirmed, thus it was never sending the coinbase
from that block to the mining participants.

pool/client.go

@@ -612,9 +612,9 @@ func (c *Client) handleSubmitWorkRequest(ctx context.Context, req *Request, allo
 		return errs.PoolError(errs.WorkRejected, desc)
 	}
 
-	// Create accepted work if the work submission is accepted
-	// by the mining node.
-	work := NewAcceptedWork(powHash.String(), header.PrevBlock.String(),
+	// Create accepted work if the work submission is accepted by the mining
+	// node.
+	work := NewAcceptedWork(header.BlockHash().String(), header.PrevBlock.String(),
 		header.Height, c.FetchAccountID(), miner)
 	err = c.cfg.db.persistAcceptedWork(work)
 	if err != nil {