Merge #16521: rpc: Use the default maxfeerate value as BTC/kB

2dfd6834ef test: Add test for default maxfeerate in sendrawtransaction (Joonmo Yang)
261843e4be wallet/rpc: Use the default maxfeerate value as BTC/kB (Joonmo Yang)

Pull request description:

  Fixes https://github.com/bitcoin/bitcoin/issues/16382

  This patch tries to treat `maxfeerate` in sendrawtransaction/testmempoolaccept RPC as a rate(BTC/kB) instead of an absolute value(BTC).
  The included test case checks if the new behavior works correctly, by using the transaction with an absolute fee of ~0.02BTC, where the fee rate is ~0.2BTC/kB.
  This test should be failing if the default `maxfeerate` is 0.1BTC, but pass if the default value is 0.1BTC/kB

ACKs for top commit:
  laanwj:
    ACK 2dfd6834ef (ACKs by Sjors and MarcoFalke above for trivially different code)

Tree-SHA512: a1795bffe8a182acef8844797955db1f60bb0c0ded97148f3572dc265234d5219271a3a7aa0b6418a43f73b2b2720ef7412ba169c99bb1cdcac52051f537d6af

test/functional/mempool_accept.py

@@ -183,6 +183,7 @@ class MempoolAcceptanceTest(BitcoinTestFramework):
         self.check_mempool_result(
             result_expected=[{'txid': tx.rehash(), 'allowed': True}],
             rawtxs=[tx.serialize().hex()],
+            maxfeerate=0,
         )
 
         self.log.info('A transaction with no outputs')