channeld: Code to implement splicing

Update the lightningd <-> channeld interface with lots of new commands to needed to facilitate spicing.

Implement the channeld splicing protocol leveraging the interactivetx protocol.

Implement lightningd’s channel_control to support channeld in its splicing efforts.

Changelog-Added: Added the features to enable splicing & resizing of active channels.

tests/test_splicing.py

@@ -0,0 +1,35 @@
+from fixtures import *  # noqa: F401,F403
+import os
+import pytest
+import unittest
+
+
+@unittest.skipIf(os.environ.get("EXPERIMENTAL_SPLICING", '0') != '1', "Need experimental splicing turned on")
+@pytest.mark.openchannel('v2')
+def test_splice(node_factory, bitcoind):
+    l1, l2 = node_factory.line_graph(2, fundamount=1000000, wait_for_announce=True)
+
+    chan_id = l1.get_channel_id(l2)
+
+    # add extra sats to pay fee
+    funds_result = l1.rpc.fundpsbt("109000sat", "slow", 166, excess_as_change=True)
+
+    result = l1.rpc.splice_init(chan_id, 100000, funds_result['psbt'])
+    result = l1.rpc.splice_update(chan_id, result['psbt'])
+    result = l1.rpc.signpsbt(result['psbt'])
+    result = l1.rpc.splice_signed(chan_id, result['signed_psbt'])
+
+    l2.daemon.wait_for_log(r'CHANNELD_NORMAL to CHANNELD_AWAITING_SPLICE')
+    l1.daemon.wait_for_log(r'CHANNELD_NORMAL to CHANNELD_AWAITING_SPLICE')
+
+    mempool = bitcoind.rpc.getrawmempool(True)
+    assert len(list(mempool.keys())) == 1
+    assert result['txid'] in list(mempool.keys())
+
+    bitcoind.generate_block(6, wait_for_mempool=1)
+
+    l2.daemon.wait_for_log(r'CHANNELD_AWAITING_SPLICE to CHANNELD_NORMAL')
+    l1.daemon.wait_for_log(r'CHANNELD_AWAITING_SPLICE to CHANNELD_NORMAL')
+
+    inv = l2.rpc.invoice(10**2, '3', 'no_3')
+    l1.rpc.pay(inv['bolt11'])