Add checking of fee

[Velnbur]

Situation

For development purposes, we still haven't implemented any solution for the coordinator to get profit, or at least prevent it from ether starving.

Objective

Implement for coordinator to check if the participant paid the fee for the coordinator.

Consideration

This issue depends on coin-shuffle/contracts#5

Acceptance

• Service has a method for showing the address to which you need to pay fee;
• Service will check the amount given by the participant's UTXOs as fees;
• Service will reject the transaction after the shuffle process if participants haven't paid enough fee;