Allow feerate of bitcoin transactions to be bumped up (RBF)

[binarybaron]

We should allow users to bump up the feerate if the mempool is too clogged up otherwise some transactions (e.g refund, cancel) might never be confirmed.

However, there are a few issues with this:

1. We can't bump up the lock transaction fee because we always use all the funds in the internal wallet which means we have no funds left to increase the fee. We can't decrease the amount either because we have agreed upon the amount with the ASB.
2. We can't bump the cancel transaction fee because the transaction is signed at swap setup and can't be changed later without cooperation from the ASB.
3. Same as 2. goes for the refund transaction