wallet: remove fetch pre-selected-inputs responsibility from SelectCoins

so if there is an error in any of the pre-set coins, we can fail right away
without computing the wallet available coins set (calling `AvailableCoins`)
which is a slow operation as it goes through the entire wallet's txes map.

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And to make the Coin Selection flow cleared, have decoupled SelectCoins in two functions:

1) AutomaticCoinSelection.
2) SelectCoins.

1) AutomaticCoinSelection:
   Receives a set of coins and selects the best subset of them to
   cover the target amount.

2) SelectCoins
   In charge of select all the user manually selected coins first ("pre-set inputs"), and
   if coin_control 'm_allow_other_inputs=true', call 'AutomaticCoinSelection' to select a
   subset of coins owned by the wallet to cover for the target - preset_inputs.total_amount
   remaining value.

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