This is largely a cherry pick of #6755 with some additional fixups added where detected.
This change moves the blockchain package to a package called blocksync. Additionally, it renames the relevant uses of the term `fastsync` to `blocksync`.
closes: #9227
#### PR checklist
- [ ] Tests written/updated, or no tests needed
- [x] `CHANGELOG_PENDING.md` updated, or no changelog entry needed
- [x] Updated relevant documentation (`docs/`) and code comments, or no
documentation updates needed
Closes#5291. Adds a randomized testnet generator. Nightly CI job will be submitted separately. A few of the testnets can be a bit flaky, even after disabling known-faulty behavior and making minor tweaks, and the larger networks may be too resource-intensive to run in CI - this will be optimized separately.
