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

account_manager/src/validator/mod.rs

@@ -3,6 +3,7 @@ pub mod deposit;
 pub mod import;
 pub mod list;
 pub mod recover;
+pub mod slashing_protection;
 
 use crate::VALIDATOR_DIR_FLAG;
 use clap::{App, Arg, ArgMatches};
@@ -33,6 +34,7 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
         .subcommand(import::cli_app())
         .subcommand(list::cli_app())
         .subcommand(recover::cli_app())
+        .subcommand(slashing_protection::cli_app())
 }
 
 pub fn cli_run<T: EthSpec>(matches: &ArgMatches, env: Environment<T>) -> Result<(), String> {
@@ -50,6 +52,9 @@ pub fn cli_run<T: EthSpec>(matches: &ArgMatches, env: Environment<T>) -> Result<
         (import::CMD, Some(matches)) => import::cli_run(matches, validator_base_dir),
         (list::CMD, Some(_)) => list::cli_run(validator_base_dir),
         (recover::CMD, Some(matches)) => recover::cli_run(matches, validator_base_dir),
+        (slashing_protection::CMD, Some(matches)) => {
+            slashing_protection::cli_run(matches, env, validator_base_dir)
+        }
         (unknown, _) => Err(format!(
             "{} does not have a {} command. See --help",
             CMD, unknown