Allow to asynchronously generate PDFs

[yhabteab]

The current implementation of HeadlessChrome::toPdf() always assumes that it controls the event loop instance, i.e. HeadlessChrome creates and starts the event loop manually. This may work for most use cases as they are mostly triggered via Icinga Web, but not if you want to generate PDFs using a daemon. Since our scheduler uses the same global event instance, it is unfavourable to call Factory::create() over again occasionally.

refs https://github.com/Icinga/icingaweb2-module-reporting/issues/229