## Issue Addressed
Closes#1891Closes#1784
## Proposed Changes
Implement checkpoint sync for Lighthouse, enabling it to start from a weak subjectivity checkpoint.
## Additional Info
- [x] Return unavailable status for out-of-range blocks requested by peers (#2561)
- [x] Implement sync daemon for fetching historical blocks (#2561)
- [x] Verify chain hashes (either in `historical_blocks.rs` or the calling module)
- [x] Consistency check for initial block + state
- [x] Fetch the initial state and block from a beacon node HTTP endpoint
- [x] Don't crash fetching beacon states by slot from the API
- [x] Background service for state reconstruction, triggered by CLI flag or API call.
Considered out of scope for this PR:
- Drop the requirement to provide the `--checkpoint-block` (this would require some pretty heavy refactoring of block verification)
Co-authored-by: Diva M <divma@protonmail.com>
## Issue Addressed
None
## Proposed Changes
* Correct typo in one comment, elaborate some others.
* Add asserts to ensure comments match code.
* Eliminate one unnecessary `clone`.
## Additional Info
None
## Issue Addressed
N/A
## Proposed Changes
This PR limits the length of the stream received by the snappy decoder to be the maximum allowed size for the received rpc message type. Also adds further checks to ensure that the length specified in the rpc [encoding-dependent header](https://github.com/ethereum/eth2.0-specs/blob/dev/specs/phase0/p2p-interface.md#encoding-strategies) is within the bounds for the rpc message type being decoded.
## Issue Addressed
#1421
## Proposed Changes
Bounding the error_message that can be returned for RPC domain errors
Co-authored-by: Age Manning <Age@AgeManning.com>
