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

Cargo.lock

@@ -2074,6 +2074,7 @@ version = "0.1.0"
 dependencies = [
  "eth2_keystore",
  "eth2_wallet",
+ "lockfile",
  "tempfile",
 ]
 
@@ -2273,6 +2274,16 @@ dependencies = [
  "percent-encoding 2.1.0",
 ]
 
+[[package]]
+name = "fs2"
+version = "0.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9564fc758e15025b46aa6643b1b77d047d1a56a1aea6e01002ac0c7026876213"
+dependencies = [
+ "libc",
+ "winapi 0.3.9",
+]
+
 [[package]]
 name = "fuchsia-cprng"
 version = "0.1.1"
@@ -3791,6 +3802,14 @@ dependencies = [
  "scopeguard",
 ]
 
+[[package]]
+name = "lockfile"
+version = "0.1.0"
+dependencies = [
+ "fs2",
+ "tempdir",
+]
+
 [[package]]
 name = "log"
 version = "0.3.9"
@@ -7387,6 +7406,7 @@ dependencies = [
  "libsecp256k1",
  "lighthouse_metrics",
  "lighthouse_version",
+ "lockfile",
  "logging",
  "parking_lot 0.11.1",
  "rand 0.7.3",
@@ -7419,8 +7439,10 @@ version = "0.1.0"
 dependencies = [
  "bls",
  "deposit_contract",
+ "derivative",
  "eth2_keystore",
  "hex",
+ "lockfile",
  "rand 0.7.3",
  "rayon",
  "slog",