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.

----------------------

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.

test/functional/wallet_send.py

@@ -508,7 +508,7 @@ class WalletSendTest(BitcoinTestFramework):
         ext_utxo = ext_fund.listunspent(addresses=[addr])[0]
 
         # An external input without solving data should result in an error
-        self.test_send(from_wallet=ext_wallet, to_wallet=self.nodes[0], amount=15, inputs=[ext_utxo], add_inputs=True, psbt=True, include_watching=True, expect_error=(-4, "Insufficient funds"))
+        self.test_send(from_wallet=ext_wallet, to_wallet=self.nodes[0], amount=15, inputs=[ext_utxo], add_inputs=True, psbt=True, include_watching=True, expect_error=(-4, "Not solvable pre-selected input COutPoint(%s, %s)" % (ext_utxo["txid"][0:10], ext_utxo["vout"])))
 
         # But funding should work when the solving data is provided
         res = self.test_send(from_wallet=ext_wallet, to_wallet=self.nodes[0], amount=15, inputs=[ext_utxo], add_inputs=True, psbt=True, include_watching=True, solving_data={"pubkeys": [addr_info['pubkey']], "scripts": [addr_info["embedded"]["scriptPubKey"], addr_info["embedded"]["embedded"]["scriptPubKey"]]})