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/Cargo.toml

@@ -33,7 +33,8 @@ slog-term = "2.6.0"
 tokio = { version = "0.2.22", features = ["time"] }
 futures = { version = "0.3.5", features = ["compat"] }
 dirs = "3.0.1"
-directory = {path = "../common/directory"}
+directory = { path = "../common/directory" }
+lockfile = { path = "../common/lockfile" }
 logging = { path = "../common/logging" }
 environment = { path = "../lighthouse/environment" }
 parking_lot = "0.11.0"