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>

beacon_node/src/cli.rs

@@ -54,7 +54,8 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
         .arg(
             Arg::with_name("shutdown-after-sync")
                 .long("shutdown-after-sync")
-                .help("Shutdown beacon node as soon as sync is completed")
+                .help("Shutdown beacon node as soon as sync is completed. Backfill sync will \
+                       not be performed before shutdown.")
                 .takes_value(false),
         )
         .arg(
@@ -479,12 +480,46 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
             Arg::with_name("wss-checkpoint")
                 .long("wss-checkpoint")
                 .help(
-                    "Used to input a Weak Subjectivity State Checkpoint in `block_root:epoch_number` format,\
-                     where block_root is an '0x' prefixed 32-byte hex string and epoch_number is an integer."
+                    "Specify a weak subjectivity checkpoint in `block_root:epoch` format to verify \
+                     the node's sync against. The block root should be 0x-prefixed. Note that this \
+                     flag is for verification only, to perform a checkpoint sync from a recent \
+                     state use --checkpoint-sync-url."
                 )
                 .value_name("WSS_CHECKPOINT")
                 .takes_value(true)
         )
+        .arg(
+            Arg::with_name("checkpoint-state")
+                .long("checkpoint-state")
+                .help("Set a checkpoint state to start syncing from. Must be aligned and match \
+                       --checkpoint-block. Using --checkpoint-sync-url instead is recommended.")
+                .value_name("STATE_SSZ")
+                .takes_value(true)
+                .requires("checkpoint-block")
+        )
+        .arg(
+            Arg::with_name("checkpoint-block")
+                .long("checkpoint-block")
+                .help("Set a checkpoint block to start syncing from. Must be aligned and match \
+                       --checkpoint-state. Using --checkpoint-sync-url instead is recommended.")
+                .value_name("BLOCK_SSZ")
+                .takes_value(true)
+                .requires("checkpoint-state")
+        )
+        .arg(
+            Arg::with_name("checkpoint-sync-url")
+                .long("checkpoint-sync-url")
+                .help("Set the remote beacon node HTTP endpoint to use for checkpoint sync.")
+                .value_name("BEACON_NODE")
+                .takes_value(true)
+                .conflicts_with("checkpoint-state")
+        )
+        .arg(
+            Arg::with_name("reconstruct-historic-states")
+                .long("reconstruct-historic-states")
+                .help("After a checkpoint sync, reconstruct historic states in the database.")
+                .takes_value(false)
+        )
         .arg(
             Arg::with_name("validator-monitor-auto")
                 .long("validator-monitor-auto")