Application that runs EventsDispatcher can safely use also cache , which
greatly improve performance. This is because cache then is properly
updated/invalidated when needed.
Instead of modifying each application to explicitly enable cache based
on this simple rule, make it implicit when EventsDispatcher is created.
Do not enable caching when EventsDispatcher is created only temporarily
in wait_for_domain_shutdown.
QubesOS/qubes-issues#3293
Do not close event loop in utility function - handle it only in main().
For this reason, change appropriate functions to coroutines.
FixesQubesOS/qubes-issues#2865
