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/cli.rs

@@ -37,11 +37,15 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                        nodes using the same key. Automatically enabled unless `--strict` is specified",
         ))
         .arg(
-            Arg::with_name("strict-lockfiles")
-            .long("strict-lockfiles")
+            Arg::with_name("delete-lockfiles")
+            .long("delete-lockfiles")
             .help(
-                "If present, do not load validators that are guarded by a lockfile. Note: for \
-                Eth2 mainnet, this flag will likely be removed and its behaviour will become default."
+                "If present, ignore and delete any keystore lockfiles encountered during start up. \
+                This is useful if the validator client did not exit gracefully on the last run. \
+                WARNING: lockfiles help prevent users from accidentally running the same validator \
+                using two different validator clients, an action that likely leads to slashing. \
+                Ensure you are certain that there are no other validator client instances running \
+                that might also be using the same keystores."
             )
         )
         .arg(