Implement slashing protection interchange format (#1544)

## Issue Addressed

Implements support for importing and exporting the slashing protection DB interchange format described here:

https://hackmd.io/@sproul/Bk0Y0qdGD

Also closes #1584 

## Proposed Changes

* [x] Support for serializing and deserializing the format
* [x] Support for importing and exporting Lighthouse's database
* [x] CLI commands to invoke import and export
* [x] Export to minimal format (required when a minimal format has been previously imported)
* [x] Tests for export to minimal (utilising mixed importing and attestation signing?)
* [x] Tests for import/export of complete format, and import of minimal format
* [x] ~~Prevent attestations with sources less than our max source (Danny's suggestion). Required for the fake attestation that we put in for the minimal format to block attestations from source 0.~~
* [x] Add the concept of a "low watermark" for compatibility with the minimal format

Bonus!

* [x] A fix to a potentially nasty bug involving validators getting re-registered each time the validator client ran! Thankfully, the ordering of keys meant that the validator IDs used for attestations and blocks remained stable -- otherwise we could have had some slashings on our hands! 😱
* [x] Tests to confirm that this bug is indeed vanquished

validator_client/src/config.rs

@@ -10,8 +10,6 @@ use std::path::PathBuf;
 use types::GRAFFITI_BYTES_LEN;
 
 pub const DEFAULT_HTTP_SERVER: &str = "http://localhost:5052/";
-/// Path to the slashing protection database within the datadir.
-pub const SLASHING_PROTECTION_FILENAME: &str = "slashing_protection.sqlite";
 
 /// Stores the core configuration for this validator instance.
 #[derive(Clone, Serialize, Deserialize)]

validator_client/src/validator_store.rs

@@ -1,10 +1,8 @@
 use crate::{
-    config::{Config, SLASHING_PROTECTION_FILENAME},
-    fork_service::ForkService,
-    initialized_validators::InitializedValidators,
+    config::Config, fork_service::ForkService, initialized_validators::InitializedValidators,
 };
 use parking_lot::RwLock;
-use slashing_protection::{NotSafe, Safe, SlashingDatabase};
+use slashing_protection::{NotSafe, Safe, SlashingDatabase, SLASHING_PROTECTION_FILENAME};
 use slog::{crit, error, warn, Logger};
 use slot_clock::SlotClock;
 use std::marker::PhantomData;