This is needed to all multi-channel splices. When channeld can return the signatures to the user (based on signing order precedent), it now does from splice_update.
Additionally, we move sending of the initial psbt from splice_init down to splice_update. This is also necessary for correct psbt diff detection during multi-channel splices.
Changelog-Changed: splice_update can in some cases now return the remotely partiall signed psbt to the user, if so `signtures_secured` will be true.
Add checking for and sending tx_abort to channeld.
When receiving it we first ACK it back, send a request to restart to lightningd, and then shutdown channeld.
We also must update the splice tests that relied on reconnect checks for splice warnings (as some are now tx_aborts instead).
Adds tests for when the connection fails during
1) splice tx_signature
2) splice commitment_signed
Fleshed out the reestablish flow for these two cases and implemented the fixes to make these reestablish flows work.
Part of this work required changing commit process for splices: Now we send a single commit_part for the splice where previously we sent all commits, and accordingly, we no longer revoke in response.
Changelog-Fixed: Implemented splicing restart logic for tx_signature and commitment_signed. Splice commitments are reworked in a manner incompatible with the last version.
