[Merged by Bors] - Refine and test slashing protection semantics

[michaelsproul]

Issue Addressed

Closes #1873

Proposed Changes

Fixes the bug in slashing protection import (#1873) by pruning the database upon import.

Also expands the test generator to cover this case and a few others which are under discussion here:

https://ethereum-magicians.org/t/eip-3076-validator-client-interchange-format-slashing-protection/4883

Additional Info

Depending on the outcome of the discussion on Eth Magicians, we can either wait for consensus before merging, or merge our preferred solution and patch things later.

[michaelsproul]

This is now ready for review.

Guide for reviewers: the most important parts of this PR are the changes to import_interchange_info. The main changes are:

• Prune the signed_blocks and signed_attestations to the low watermark from the interchange file in order to solve the bug raised in Fix bug in slashing protection import #1873.
• Allow partial import of an interchange file: ignore records that appear slashable wrt to the existing database, or the interchange file itself. This still leaves some sharp corners on the UX for re-imports, but they will have to be addressed in subsequent work. I've taken the approach of making the code as safe as possible while changing as little as possible, i.e. not introducing drastic changes just before mainnet. Some other interchange implementers are pursuing an import strategy that allows seemingly slashable data to be imported (with better UX + less manual intervention), but I'm hesitant to put this into Lighthouse without first thoroughly testing all of the corner cases.
• Update the CLI command to output some summary information, so the user has some idea of the state of their database after the import, and direct them to a page in the Lighthouse book (and Discord) if they hit one of the sharp corners on import.

You can likely skim over several hundred lines of changes to the test generator and test runner.

[paulhauner]

Looks great, I read thought it twice and didn't find anything. I did skip the tests though.

Merge at will :)

[michaelsproul]

bors r+

[bors]

Pull request successfully merged into unstable.

Build succeeded:

• check-consensus
• state-transition-vectors-ubuntu
• cargo-udeps
• cargo-audit
• arbitrary-check
• release-tests-ubuntu
• dockerfile-ubuntu
• no-eth1-simulator-ubuntu
• clippy
• ef-tests-ubuntu
• eth1-simulator-ubuntu
• cargo-fmt
• debug-tests-ubuntu
• check-benchmarks