Merge #16524: Wallet: Disable -fallbackfee by default

ea4cc3a7b3 Truly decouple wallet from chainparams for -fallbackfee (Jorge Timón)

Pull request description:

  Before it was 0 by default for main and 20000 for test and regtest.
  Now it is 0 by default for all chains, thus there's no need to call Params().

  Also now the default for main is properly documented.

  Suggestion for release notes:

  -fallbackfee was 0 (disabled) by default for the main chain, but 20000 by default for the test chains. Now it is 0 by default for all chains. Testnet and regtest users will have to add fallbackfee=20000 to their configuration if they weren't setting it and they want it to keep working like before.

  Should I propose them to the wiki for the release notes or only after merge?

  For more context, see https://github.com/bitcoin/bitcoin/pull/16402#issuecomment-515701042

ACKs for top commit:
  MarcoFalke:
    ACK ea4cc3a7b3

Tree-SHA512: fdfaba5d813da4221e405e0988bef44f3856d10f897a94f9614386d14b7716f4326ab8a6646e26d41ef3f4fa61b936191e216b1b605e9ab0520b0657fc162e6c

test/functional/test_framework/util.py

@@ -307,6 +307,7 @@ def initialize_datadir(dirname, n, chain):
         f.write("[{}]\n".format(chain_name_conf_section))
         f.write("port=" + str(p2p_port(n)) + "\n")
         f.write("rpcport=" + str(rpc_port(n)) + "\n")
+        f.write("fallbackfee=0.0002\n")
         f.write("server=1\n")
         f.write("keypool=1\n")
         f.write("discover=0\n")