client/core: try a fresh fee rate for the init

[chappjc]

Resolves #1489

When creating the swap transaction, see if a more current fee rate
should be used instead of the prescribed swap rate. This can be
particularly helpful for the taker because their swap may be a
long time after the match is made because the maker's swap must
be fully confirmed first.

If the current rate is swapRate < currentRate <= MaxFeeRate, then
use the current rate for the swap transaction.

We can also consider protocol changes where taker must get a new rate from server, and that will be enforced separately.
This current change is helpful even with that because it guards against server giving rates that are too low for whatever reason.