Fix validator lockfiles (#1586)

## Issue Addressed

- Resolves #1313 

## Proposed Changes

Changes the way we start the validator client and beacon node to ensure that we cleanly drop the validator keystores (which therefore ensures we cleanup their lockfiles).

Previously we were holding the validator keystores in a tokio task that was being forcefully killed (i.e., without `Drop`). Now, we hold them in a task that can gracefully handle a shutdown.

Also, switches the `--strict-lockfiles` flag to `--delete-lockfiles`. This means two things:

1. We are now strict on lockfiles by default (before we weren't).
1. There's a simple way for people delete the lockfiles if they experience a crash.

## Additional Info

I've only given the option to ignore *and* delete lockfiles, not just ignore them. I can't see a strong need for ignore-only but could easily add it, if the need arises.

I've flagged this as `api-breaking` since users that have lockfiles lingering around will be required to supply `--delete-lockfiles` next time they run.

validator_client/src/config.rs

@@ -24,8 +24,8 @@ pub struct Config {
     /// If true, the validator client will still poll for duties and produce blocks even if the
     /// beacon node is not synced at startup.
     pub allow_unsynced_beacon_node: bool,
-    /// If true, refuse to unlock a keypair that is guarded by a lockfile.
-    pub strict_lockfiles: bool,
+    /// If true, delete any validator keystore lockfiles that would prevent starting.
+    pub delete_lockfiles: bool,
     /// If true, don't scan the validators dir for new keystores.
     pub disable_auto_discover: bool,
     /// Graffiti to be inserted everytime we create a block.
@@ -46,7 +46,7 @@ impl Default for Config {
             secrets_dir,
             http_server: DEFAULT_HTTP_SERVER.to_string(),
             allow_unsynced_beacon_node: false,
-            strict_lockfiles: false,
+            delete_lockfiles: false,
             disable_auto_discover: false,
             graffiti: None,
         }
@@ -77,7 +77,7 @@ impl Config {
         }
 
         config.allow_unsynced_beacon_node = cli_args.is_present("allow-unsynced");
-        config.strict_lockfiles = cli_args.is_present("strict-lockfiles");
+        config.delete_lockfiles = cli_args.is_present("delete-lockfiles");
         config.disable_auto_discover = cli_args.is_present("disable-auto-discover");
 
         if let Some(secrets_dir) = parse_optional(cli_args, "secrets-dir")? {