FIX: Outbound gas limit to be a feeRate not feeAmount

Current

Nodes are told to spend a maximum in feeAmount, but if the tx size changes at all, this can drastically increase/decrease the final fee rate and change priority untentionally.

Example:

"medianfee": 15405,
	"mediantime": 1604450778,
	"mediantxsize": 564,

median feeRate is then 15405/564 = 27sats/byte

For a typical (1 in, 2 out), the size is 148 * 1 + 10 + 10 * 2 = 178 bytes, so if told to spend up to 15405 * 1.5 sats, then the node will use a feeRate of ~110 sats/byte, which is massively overspending.

The other issue, is the node could be consuming a lot of UTXOs, (like 20 in, 2 out) size: 148 * 20 + 10 + 10 * 2 = 3000 bytes feeRate = 7 sats/byte,

Which means the transaction is well below and will get delayed.

Desired

Target a feeRate.

nodes should spend 1.5 * 27 = 40 sats/byte so no matter the size of the tx, it will be correct fee rate and get processed with desired priority.