Refactor the multiple "notify" systems into a single helper

structure.

* Creates FileWatcher, in lqos_utils.
* Removes "notify" dependency from other crates.

FileWatcher is designed to watch a file. If the file doesn't exist,
then an optional callback is called - and the watcher waits,
periodically checking to see if the file has appeared yet. When the
file appears, another optional callback is executed.

Once the file exists, a `notify` system is started (it uses Linux's
`inotify` system internally) for that file. When the file changes,
the process sleeps briefly and then executes an `on_change` callback.
Further messages are then suppressed for a short period to avoid
duplicates.

All uses of notify have been updated to use this system. Errors are
handled cleanly, per ISSUE #209.

src/rust/Cargo.lock

@@ -1348,8 +1348,8 @@ dependencies = [
  "lazy_static",
  "lqos_bus",
  "lqos_config",
+ "lqos_utils",
  "nix",
- "notify",
  "parking_lot 0.12.1",
  "rocket",
  "rocket_async_compression",
@@ -1379,7 +1379,6 @@ dependencies = [
  "lqos_config",
  "lqos_sys",
  "lqos_utils",
- "notify",
  "parking_lot 0.12.1",
  "rayon",
  "serde",
@@ -1412,7 +1411,9 @@ version = "0.1.0"
 dependencies = [
  "log",
  "nix",
+ "notify",
  "serde",
+ "thiserror",
 ]
 
 [[package]]
@@ -1430,7 +1431,6 @@ dependencies = [
  "lqos_sys",
  "lqos_utils",
  "nix",
- "notify",
  "parking_lot 0.12.1",
  "rayon",
  "serde",