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/wallet_manager.rs

@@ -3,6 +3,7 @@ use crate::{
     LockedWallet,
 };
 use eth2_wallet::{bip39::Mnemonic, Error as WalletError, Uuid, Wallet, WalletBuilder};
+use lockfile::LockfileError;
 use std::collections::HashMap;
 use std::ffi::OsString;
 use std::fs::{create_dir_all, read_dir, OpenOptions};
@@ -21,10 +22,9 @@ pub enum Error {
     WalletNameUnknown(String),
     WalletDirExists(PathBuf),
     IoError(io::Error),
-    WalletIsLocked(PathBuf),
     MissingWalletDir(PathBuf),
-    UnableToCreateLockfile(io::Error),
     UuidMismatch((Uuid, Uuid)),
+    LockfileError(LockfileError),
 }
 
 impl From<io::Error> for Error {
@@ -45,6 +45,12 @@ impl From<FilesystemError> for Error {
     }
 }
 
+impl From<LockfileError> for Error {
+    fn from(e: LockfileError) -> Error {
+        Error::LockfileError(e)
+    }
+}
+
 /// Defines the type of an EIP-2386 wallet.
 ///
 /// Presently only `Hd` wallets are supported.
@@ -358,7 +364,7 @@ mod tests {
         );
 
         match LockedWallet::open(&base_dir, &uuid_a) {
-            Err(Error::WalletIsLocked(_)) => {}
+            Err(Error::LockfileError(_)) => {}
             _ => panic!("did not get locked error"),
         };
 
@@ -368,7 +374,7 @@ mod tests {
             .expect("should open wallet a after previous instance is dropped");
 
         match LockedWallet::open(&base_dir, &uuid_b) {
-            Err(Error::WalletIsLocked(_)) => {}
+            Err(Error::LockfileError(_)) => {}
             _ => panic!("did not get locked error"),
         };