Overhaul importmulti logic

Summary:
This introduces various changes to the importmulti logic:

• Instead of processing input and importing things at the same time, first process all input data and verify it, so no changes are made in case of an error.
• Verify that no superfluous information is provided (no keys or scripts that don't contribute to solvability in particular).
• Add way more sanity checks, by means of descending into all involved scripts.
  • Add release notes

This is a backport of Core PR14565

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6140