lightningd: trace JSON id prefixes through sendrawtx.

First, merge the _ahf_ and non-ahf interfaces.
Second, remove the always-NULL txs->cmd field.

Then, add optional id_prefix for bitcoind_sendrawx, so if it's
triggered by a command (e.g. "withdraw") it's shown correctly in logs.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_wallet.py

@@ -59,6 +59,9 @@ def test_withdraw(node_factory, bitcoind):
 
     out = l1.rpc.withdraw(waddr, 2 * amount)
 
+    # Side note: sendrawtransaction will trace back to withdrawl
+    l1.daemon.wait_for_log(": OUT:id=[0-9]*/cln:sendrawtransaction#[0-9]*")
+
     # Make sure bitcoind received the withdrawal
     unspent = l1.bitcoin.rpc.listunspent(0)
     withdrawal = [u for u in unspent if u['txid'] == out['txid']]