sd-event: remove earliest_index/latest_index into common part of event source objects

sd-event: remove earliest_index/latest_index into common part of event source objects

So far we used these fields to organize the earliest/latest timer event
priority queue.  In a follow-up commit we want to introduce ratelimiting
to event sources, at which point we want any kind of event source to be
able to trigger time wakeups, and hence they all need to be included in
the earliest/latest prioqs.  Thus, in preparation let's make this
generic.

No change in behaviour, just some shifting around of struct members from
the type-specific to the generic part.

(cherry picked from commit f41315fceb5208c496145cda2d6c865a5458ce44)

Related: #1819868