[kwsantiago]

BOLT 2 says a node must not send shutdown while a splice transaction is unlocked. channeld now holds our shutdown while a splice both sides have signed is pending, and sends it once the splice locks. If the peer sends shutdown first, we reply right away, since waiting on a peer that may never lock the splice would hang the close.

• channeld refuses splice_init once either side has sent shutdown. An RBF of the pending splice is still accepted while our shutdown is held.
• lightningd keeps watching the splice depth in CHANNELD_SHUTTING_DOWN, including after a restart, and handles a splice lock in that state without announcing the new scid.
• A close requested after splice signatures are exchanged stays in CHANNELD_SHUTTING_DOWN, and the splice is still broadcast.
• New dev option --dev-shutdown-during-splice sends shutdown without waiting, to test peers that do not hold it.
• New test test_splice_holds_shutdown_until_locked covers the hold, an RBF while held, a restart while held, and a mutual close of the locked splice output.

Changelog-None