Restore the default minimum fees and minimum transaction size

[yangl1996]

We changed those values in d3d8dd0 and 9440feb and they break integration tests. Specifically, d3d8dd0 breaks TestLightningNetworkDaemon/update_channel_policy because the test is waiting for a channel policy update with minHTLC to be 1000 (the original default value), while we have changed the value to 0. Reverting d3d8dd0 makes all integration tests pass.