Implement checkpoint sync (#2244)

## Issue Addressed

Closes #1891
Closes #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>

lcli/src/main.rs

@@ -16,7 +16,6 @@ mod transition_blocks;
 use clap::{App, Arg, ArgMatches, SubCommand};
 use clap_utils::parse_path_with_default_in_home_dir;
 use environment::EnvironmentBuilder;
-use log::LevelFilter;
 use parse_ssz::run_parse_ssz;
 use std::path::PathBuf;
 use std::process;
@@ -25,10 +24,7 @@ use transition_blocks::run_transition_blocks;
 use types::{EthSpec, EthSpecId};
 
 fn main() {
-    simple_logger::SimpleLogger::new()
-        .with_level(LevelFilter::Info)
-        .init()
-        .expect("Logger should be initialised");
+    env_logger::init();
 
     let matches = App::new("Lighthouse CLI Tool")
         .version(lighthouse_version::VERSION)
@@ -110,6 +106,17 @@ fn main() {
         .subcommand(
             SubCommand::with_name("pretty-ssz")
                 .about("Parses SSZ-encoded data from a file")
+                .arg(
+                    Arg::with_name("format")
+                        .short("f")
+                        .long("format")
+                        .value_name("FORMAT")
+                        .takes_value(true)
+                        .required(true)
+                        .default_value("json")
+                        .possible_values(&["json", "yaml"])
+                        .help("Output format to use")
+                )
                 .arg(
                     Arg::with_name("type")
                         .value_name("TYPE")
@@ -123,7 +130,7 @@ fn main() {
                         .takes_value(true)
                         .required(true)
                         .help("Path to SSZ bytes"),
-                ),
+                )
         )
         .subcommand(
             SubCommand::with_name("deploy-deposit-contract")