`win32_filesystem_monitor` misses first event

### Checklist

- [x] I did not find a duplicate of this bug in the Github Issues section.

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### Description

The `win32_filesystem_monitor` consistently misses firing the _first_ event after being registered.

### Steps to reproduce
1. Construct a `win32_filesystem_monitor `
2. Register a listener to the monitor
3. Register the monitor to a `message_pump`
4. Watch a directory
5. Modify something in that directory

#### Expected Behavior

The first modification is reported

#### Actual Behavior

The second modification and onward is reported.

### Extra information

* Operating System: **Windows 10**
* Compiler: **clang-8.0**