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/validator_dir/src/lib.rs

@@ -2,14 +2,12 @@
 //!
 //! - `ValidatorDir`: manages a directory containing validator keypairs, deposit info and other
 //! things.
-//! - `Manager`: manages a directory that contains multiple `ValidatorDir`.
 //!
 //! This crate is intended to be used by the account manager to create validators and the validator
 //! client to load those validators.
 
 mod builder;
 pub mod insecure_keys;
-mod manager;
 mod validator_dir;
 
 pub use crate::validator_dir::{
@@ -20,4 +18,3 @@ pub use builder::{
     Builder, Error as BuilderError, ETH1_DEPOSIT_DATA_FILE, VOTING_KEYSTORE_FILE,
     WITHDRAWAL_KEYSTORE_FILE,
 };
-pub use manager::{Error as ManagerError, Manager};