tracking: remaining cleanup to address before merging `0-19-staging` branch into master

[guggero]

This issue tracks all cleanup that is necessary to address before we can merge the 0-19-staging branch into master.

• https://github.com/lightningnetwork/lnd/pull/8509#discussion_r1606975757
• https://github.com/lightningnetwork/lnd/pull/8509#discussion_r1588119871
• https://github.com/lightningnetwork/lnd/pull/8509#issuecomment-2120858347
• https://github.com/lightningnetwork/lnd/pull/8660#discussion_r1608641948
• https://github.com/lightningnetwork/lnd/pull/8660#discussion_r1609843872
• https://github.com/lightningnetwork/lnd/pull/8660#discussion_r1606720575
• https://github.com/lightningnetwork/lnd/pull/8632#discussion_r1612091616
• https://github.com/lightningnetwork/lnd/pull/8771#discussion_r1616920049
• revert itest change in https://github.com/lightningnetwork/lnd/pull/8908/commits/4f2c75f62047579f249ed5914042a0cc6337f342 to properly test amount modification
  • Need to use invoice acceptor to be able to pull this off, so create invoice for amount X, actually send X*2 (which'll be in the MPP amount to expect), intercept HTLC to modify it back to X then use invoice acceptor to settle with just X and ignore MPP total amount)