Use OS file locks in validator client (#1958)

## Issue Addressed

Closes #1823

## Proposed Changes

* Use OS-level file locking for validator keystores, eliminating problems with lockfiles lingering after ungraceful shutdowns (`SIGKILL`, power outage). I'm using the `fs2` crate because it's cross-platform (unlike `file-lock`), and it seems to have the most downloads on crates.io.
* Deprecate + disable `--delete-lockfiles` CLI param, it's no longer necessary
* Delete the `validator_dir::Manager`, as it was mostly dead code and was only used in the `validator list` command, which has been rewritten to read the validator definitions YAML instead.

## Additional Info

Tested on:

- [x] Linux
- [x] macOS
- [x] Docker Linux
- [x] Docker macOS
- [ ] Windows

validator_client/src/http_api/mod.rs

@@ -352,11 +352,14 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
                             ))
                         })?;
 
+                    // Drop validator dir so that `add_validator_keystore` can re-lock the keystore.
+                    let voting_keystore_path = validator_dir.voting_keystore_path();
+                    drop(validator_dir);
                     let voting_password = body.password.clone();
 
                     let validator_def = tokio::runtime::Handle::current()
                         .block_on(validator_store.add_validator_keystore(
-                            validator_dir.voting_keystore_path(),
+                            voting_keystore_path,
                             voting_password,
                             body.enable,
                         ))