Respect `mod_unique_id` and refactor `OC_Request::getRequestId`

When `mod_unique_id` is enabled the ID generated by it will be used for logging. This allows for correlation of the Apache logs and the ownCloud logs.

Testplan:

- [ ] When `mod_unique_id` is enabled the request ID equals the one generated by `mod_unique_id`.
- [ ] When `mod_unique_id` is not available the request ID is a 20 character long random string
- [ ] The generated Id is stable over the lifespan of one request

Changeset looks a little bit larger since I had to adjust every unit test using the HTTP\Request class for proper DI.

Fixes https://github.com/owncloud/core/issues/13366