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/Cargo.toml

@@ -19,6 +19,8 @@ rayon = "1.4.1"
 tree_hash = "0.1.1"
 slog = { version = "2.5.2", features = ["max_level_trace", "release_max_level_trace"] }
 hex = "0.4.2"
+derivative = "2.1.1"
+lockfile = { path = "../lockfile" }
 
 [dev-dependencies]
 tempfile = "3.1.0"