[0.5 XMR] Failed to broadcast Bitcoin redeem transaction: mempool min fee not met

[binarybaron]

Describe the bug
If after the tx_redeem_fee is negotiated at swap start, the mempool fills up dramatically, electrum might reject the tx redeem transaction due to the fee being too low. This results in an error such as:

Failed to broadcast Bitcoin redeem transaction XXXX: Electrum client error: Electrum server error: {"code":2,"message":"mempool min fee not met, 1314 < 1519"}

The swap will not continue because Alice cannot redeem her Bitcoin.

We should consider allowing Bob and Alice to update the redeem fees later to reflect the current status of the mempool.

Lost/trapped Funds
Yes

[binarybaron]

To clarify: Only Alice should be able to propose a change in the fee

[binarybaron]

Relate to #1635