fix timeout handling
* Despite what is documented in sd_bus_get_timeout(3), the timeout
returned is actually an absolute time point of Linux's CLOCK_MONOTONIC
clock. Hence, we first have to subtract the current time from the
timeout in order to get a relative time that can be passed to poll.
* For async call timeouts to reliably work, we need a way to notify the
event loop of a connection that is currently blocked waiting in poll.
I.e. assume the event loop thread entered poll with a timeout set to
T1. Afterwards, the main thread starts an async call C with a timeout
T2 < T1. In order for C to be canceled after its timeout T1 has
elapsed, we have to be able to notify the event loop so that it can
update its poll data.
Co-authored-by: Urs Ritzmann <ursritzmann@protonmail.ch>
Co-authored-by: Lukasz Marcul <lukasz.marcul@onemeter.com>
