Add an admin port endpoint for removing units from the unit cache
Summary:
Add a new `/invalidate-units` endpoint to the admin interface, to enable us to measure bytecode compilation performance on collections of dependent files.
This endpoint can be used as follows, assuming a local HHVM with admin port 8003:
```
$ curl 'http://localhost:8003/invalidate-units?path=/my/file.php&path=/my/other/file.php'
```
Reviewed By: paulbiss
Differential Revision:
D15149555
fbshipit-source-id:
5e30b46aa6e5e61c1f8a1bbcaa1a5fda3ca1abf6