This PR takes advantage of the new fee estimation feature that will potentially give lower estimates if recent market conditions warrant it. The logic used here is that any time a transaction signals opt-in-RBF and uses automatic fee estimation then it will use the non-conservative estimate. Transactions which do not signal opt-in-RBF will still use the default conservative estimate.
In a nutshell conservative estimates require that your fee rate would meet the necessary confirmation threshold for double your target at longer time horizons as well. This reduces the likelihood that you place a transaction just as the market is starting to get busy again that ends up being stuck for a very long time.
This PR also allows the specification of transaction confirmation target and whether the estimate should be conservative or not on a per-RPC call basis for sendtoaddress, sendmany, bumpfee, and fundrawtransaction using optional named arguments.
Left as an exercise to the reader is adding this to GUI functions to send transactions and bump fee.