fees: Always round up fee calculated from a feerate

When calculating the fee for a given tx size from a fee rate, we should
always round up to the next satoshi. Otherwise, if we round down (via
truncation), the calculated fee may result in a fee with a feerate
slightly less than targeted.

This is particularly important for coin selection as a slightly lower
feerate than expected can result in a variety of issues.

test/functional/wallet_keypool.py

@@ -179,7 +179,7 @@ class KeyPoolTest(BitcoinTestFramework):
         assert_equal("psbt" in res, True)
 
         # create a transaction without change at the maximum fee rate, such that the output is still spendable:
-        res = w2.walletcreatefundedpsbt(inputs=[], outputs=[{destination: 0.00010000}], options={"subtractFeeFromOutputs": [0], "feeRate": 0.0008824})
+        res = w2.walletcreatefundedpsbt(inputs=[], outputs=[{destination: 0.00010000}], options={"subtractFeeFromOutputs": [0], "feeRate": 0.0008823})
         assert_equal("psbt" in res, True)
         assert_equal(res["fee"], Decimal("0.00009706"))