journald: use the event loop dispatch timestamp for journal entries

journald: use the event loop dispatch timestamp for journal entries

Let's use the earliest linearized event timestamp for journal entries we have:
the event dispatch timestamp from the event loop, instead of requerying the
timestamp at the time of writing.

This makes the time a bit more accurate, allows us to query the kernel time one
time less per event loop, and also makes sure we always use the same timestamp
for both attempts to write an entry to a journal file.