reimburseRedeem - implement reimbursement functionality

[alexeiZamyatin]

Required checks:

• Redeem must be free! i.e., vault cannot charged a fee for this
• User specifies bitcoin transaction fee? Challenge: if too low, tx never included. Who is to blame then? Problem!
• We might need an oracle for current bitcoin transaction fees.

--> if BTC relay is used frequently, we can use verified TX as an estimate