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/src/cli.rs

@@ -57,12 +57,7 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
             Arg::with_name("delete-lockfiles")
             .long("delete-lockfiles")
             .help(
-                "If present, ignore and delete any keystore lockfiles encountered during start up. \
-                This is useful if the validator client did not exit gracefully on the last run. \
-                WARNING: lockfiles help prevent users from accidentally running the same validator \
-                using two different validator clients, an action that likely leads to slashing. \
-                Ensure you are certain that there are no other validator client instances running \
-                that might also be using the same keystores."
+                "DEPRECATED. This flag does nothing and will be removed in a future release."
             )
         )
         .arg(