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

account_manager/src/validator/list.rs

@@ -1,24 +1,28 @@
-use crate::VALIDATOR_DIR_FLAG;
+use account_utils::validator_definitions::ValidatorDefinitions;
 use clap::App;
 use std::path::PathBuf;
-use validator_dir::Manager as ValidatorManager;
 
 pub const CMD: &str = "list";
 
 pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
-    App::new(CMD).about("Lists the names of all validators.")
+    App::new(CMD).about("Lists the public keys of all validators.")
 }
 
 pub fn cli_run(validator_dir: PathBuf) -> Result<(), String> {
     eprintln!("validator-dir path: {:?}", validator_dir);
-    let mgr = ValidatorManager::open(&validator_dir)
-        .map_err(|e| format!("Unable to read --{}: {:?}", VALIDATOR_DIR_FLAG, e))?;
+    let validator_definitions = ValidatorDefinitions::open(&validator_dir).map_err(|e| {
+        format!(
+            "No validator definitions found in {:?}: {:?}",
+            validator_dir, e
+        )
+    })?;
 
-    for (name, _path) in mgr
-        .directory_names()
-        .map_err(|e| format!("Unable to list validators: {:?}", e))?
-    {
-        println!("{}", name)
+    for def in validator_definitions.as_slice() {
+        println!(
+            "{} ({})",
+            def.voting_public_key,
+            if def.enabled { "enabled" } else { "disabled" }
+        );
     }
 
     Ok(())