When targeting a feerate using bumpfee, call a new function that directly uses CWallet::CreateTransaction and coin control to get the desired result. This allows us to get a superset of previous behavior, with an arbitrary RBF bump of a transaction provided it passes the preconditional checks and spare confirmed utxos are available.

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<jnewbery> knapsack coin selection is the old method for doing coin selection in the wallet (pre branch-and-bound)