lightning: change derivation of funding_pubkey

Ideally, given an on-chain backup, after the remote force-closes, we should be able to spend our anchor output,
to CPFP the remote commitment tx (assuming the channel used OPTION_ANCHORS).
To spend the anchor output, we need to be able to sign with the local funding_privkey.

Previously we derived the funding_key from the channel_seed (which comes from os.urandom).
Prior to anchors, there was no use case for signing with the funding_key given a channel backup.
Now with anchors, we should make its derivation deterministic somehow, in a way so that it can
be derived given just an on-chain backup.
- one way would be to put some more data into the existing OP_RETURN
  - uses block space
  - the OP_RETURNs can be disabled via "use_recoverable_channels"
  - only the initiator can use OP_RETURNs (so what if channel is in incoming dir?)
- instead, new scheme for our funding_key:
  - we derive the funding_privkey from the lnworker root secret (derived from our bip32 seed)
  - for outgoing channels:
    - lnworker_root_secret + remote_node_id + funding_tx_nlocktime
  - for incoming channels:
    - lnworker_root_secret + remote_node_id + remote_funding_pubkey
  - a check is added to avoid reusing the same key between channels:
      not letting to user open more than one channel with the same peer in a single block
  - only the first 16 bytes of the remote_node_id are used, as the onchain backup OP_RETURNs only contain that
- as the funding_privkey cannot be derived from the channel_seed anymore, it is included in the
imported channel backups, which in turn need a new version defined
  - a wallet db upgrade is used to update already stored imported cbs
  - alternatively we could keep the imported cbs as-is, so no new version, no new funding_privkey field, as it is clearly somewhat redundant given on-chain backups can reconstruct it
    - however adding the field seems easier
      - otherwise the existing code would try to derive the funding_privkey from the channel_seed
      - also note: atm there is no field in the imported backups to distinguish anchor channels vs static-remotekey channels

tests/regtest/regtest.sh

@@ -156,6 +156,7 @@ if [[ $1 == "backup" ]]; then
     echo "alice opens channel"
     bob_node=$($bob nodeid)
     channel1=$($alice open_channel $bob_node 0.15 --password='')
+    new_blocks 1  # cannot open multiple chans with same node in same block
     $alice setconfig use_recoverable_channels False
     channel2=$($alice open_channel $bob_node 0.15 --password='')
     new_blocks 3

tests/test_lnutil.py

@@ -1086,6 +1086,7 @@ class TestLNUtil(ElectrumTestCase):
                 remote_payment_pubkey=bfh('02a1bbc818e2e88847016a93c223eb4adef7bb8becb3709c75c556b6beb3afe7bd'),
                 remote_revocation_pubkey=bfh('022f28b7d8d1f05768ada3df1b0966083b8058e1e7197c57393e302ec118d7f0ae'),
                 local_payment_pubkey=None,
+                multisig_funding_privkey=None,
             ),
             decoded_cb,
         )
@@ -1113,6 +1114,7 @@ class TestLNUtil(ElectrumTestCase):
                 remote_payment_pubkey=bfh('02a1bbc818e2e88847016a93c223eb4adef7bb8becb3709c75c556b6beb3afe7bd'),
                 remote_revocation_pubkey=bfh('022f28b7d8d1f05768ada3df1b0966083b8058e1e7197c57393e302ec118d7f0ae'),
                 local_payment_pubkey=bfh('0308d686712782a44b0cef220485ad83dae77853a5bf8501a92bb79056c9dcb25a'),
+                multisig_funding_privkey=None,
             ),
             decoded_cb,
         )