sd-event: allocate event source objects with the actually needed size

sd-event: allocate event source objects with the actually needed size

Currently we allocate fixed-size memory for event sources: the largest
any of the event source type needs. Discrepancy in the sizes needed for
the various event sources is quite major however: it's 144 bytes on
x86_64, i.e. more than two cache lines.

hence, let's be a tiny bit more careful, and allocate exactly as much as
we need, but not more.