## Issue Addressed
Closes#2286Closes#2538Closes#2342
## Proposed Changes
Part II of major slasher optimisations after #2767
These changes will be backwards-incompatible due to the move to MDBX (and the schema change) 😱
* [x] Shrink attester keys from 16 bytes to 7 bytes.
* [x] Shrink attester records from 64 bytes to 6 bytes.
* [x] Separate `DiskConfig` from regular `Config`.
* [x] Add configuration for the LRU cache size.
* [x] Add a "migration" that deletes any legacy LMDB database.
## Issue Addressed
Closes https://github.com/sigp/lighthouse/issues/2112
Closes https://github.com/sigp/lighthouse/issues/1861
## Proposed Changes
Collect attestations by validator index in the slasher, and use the magic of reference counting to automatically discard redundant attestations. This results in us storing only 1-2% of the attestations observed when subscribed to all subnets, which carries over to a 50-100x reduction in data stored 🎉
## Additional Info
There's some nuance to the configuration of the `slot-offset`. It has a profound effect on the effictiveness of de-duplication, see the docs added to the book for an explanation: 5442e695e5/book/src/slasher.md (slot-offset)
## Issue Addressed
Closes#2119
## Proposed Changes
Update the slasher schema version to v2 for the breaking changes to the config introduced in #2079. Implement a migration from v1 to v2 so that users can seamlessly upgrade from any version of Lighthouse <=1.0.5.
Users who deleted their database for v1.0.5 can upgrade to a release including this patch without any manual intervention. Similarly, any users still on v1.0.4 or earlier can now upgrade without having to drop their database.
## Issue Addressed
Closes#2048
## Proposed Changes
* Broadcast slashings when the `--slasher-broadcast` flag is provided.
* In the process of implementing this I refactored the slasher service into its own crate so that it could access the network code without creating a circular dependency. I moved the responsibility for putting slashings into the op pool into the service as well, as it makes sense for it to handle the whole slashing lifecycle.
This is an implementation of a slasher that lives inside the BN and can be enabled via `lighthouse bn --slasher`.
Features included in this PR:
- [x] Detection of attester slashing conditions (double votes, surrounds existing, surrounded by existing)
- [x] Integration into Lighthouse's attestation verification flow
- [x] Detection of proposer slashing conditions
- [x] Extraction of attestations from blocks as they are verified
- [x] Compression of chunks
- [x] Configurable history length
- [x] Pruning of old attestations and blocks
- [x] More tests
Future work:
* Focus on a slice of history separate from the most recent N epochs (e.g. epochs `current - K` to `current - M`)
* Run out-of-process
* Ingest attestations from the chain without a resync
Design notes are here https://hackmd.io/@sproul/HJSEklmPL
