Vault creation fee uses current creation fee for refund instead of creation fee paid

[shohamc1]

When a user closes their vault, it should refund 50% of the creation fee. At the moment, the 50% of the current creation fee is refunded.

ain/src/masternodes/mn_checks.cpp

Line 2858 in 0989ff6

auto feeBack = consensus.vaultCreationFee / 2;

This means that if the creation fee is changed, then users may be refunded more/less than 50% of the creation fee that they paid.

Scenario 1

• User A creates vault with 2 DFI
• Creation fee is increased to 4 DFI
• User A closes vault, receives $\frac{4}{2}$ = 2 DFI as refund, but should have received 1 DFI

Scenario 2

• User A creates vault with 2 DFI
• Creation fee is decreased to 0 DFI
• User A closes vault, receives $\frac{0}{2}$ = 0 DFI as refund, but should have received 1 DFI

The node should keep track of the creation fee paid for the vault, and return 50% of the fee paid instead.