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

common/eth2_wallet_manager/src/locked_wallet.rs

@@ -3,7 +3,7 @@ use crate::{
     Error,
 };
 use eth2_wallet::{Uuid, ValidatorKeystores, Wallet};
-use std::fs::{remove_file, OpenOptions};
+use lockfile::Lockfile;
 use std::path::{Path, PathBuf};
 
 pub const LOCK_FILE: &str = ".lock";
@@ -26,6 +26,7 @@ pub const LOCK_FILE: &str = ".lock";
 pub struct LockedWallet {
     wallet_dir: PathBuf,
     wallet: Wallet,
+    _lockfile: Lockfile,
 }
 
 impl LockedWallet {
@@ -49,20 +50,12 @@ impl LockedWallet {
             return Err(Error::MissingWalletDir(wallet_dir));
         }
 
-        let lockfile = wallet_dir.join(LOCK_FILE);
-        if lockfile.exists() {
-            return Err(Error::WalletIsLocked(wallet_dir));
-        } else {
-            OpenOptions::new()
-                .write(true)
-                .create_new(true)
-                .open(lockfile)
-                .map_err(Error::UnableToCreateLockfile)?;
-        }
+        let _lockfile = Lockfile::new(wallet_dir.join(LOCK_FILE))?;
 
         Ok(Self {
             wallet: read(&wallet_dir, uuid)?,
             wallet_dir,
+            _lockfile,
         })
     }
 
@@ -99,13 +92,3 @@ impl LockedWallet {
         Ok(keystores)
     }
 }
-
-impl Drop for LockedWallet {
-    /// Clean-up the lockfile.
-    fn drop(&mut self) {
-        let lockfile = self.wallet_dir.clone().join(LOCK_FILE);
-        if let Err(e) = remove_file(&lockfile) {
-            eprintln!("Unable to remove {:?}: {:?}", lockfile, e);
-        }
-    }
-}