journal: letting (interleaved) seqnums go

In the following scenario:
  server creates system.journal
  server creates user-1000.journal
both journals share the same seqnum_id.
Then
  server writes to user-1000.journal first,
  and server writes to system.journal a bit later,
and everything is fine.
The server then terminates (crash, reboot, rsyslog testing,
whatever), and user-1000.journal has entries which end with
a lower seqnum than system.journal. Now
  server is restarted
  server opens user-1000.journal and writes entries to it...
BAM! duplicate seqnums for the same seqnum_id.

Now, we usually don't see that happen, because system.journal
is closed last, and opened first. Since usually at least one
message is written during boot and lands in the system.journal,
the seqnum is initialized from it, and is set to a number higher
than than anything found in user journals. Nevertheless, if
system.journal is corrupted and is rotated, it can happen that
an entry is written to the user journal with a seqnum that is
a duplicate with an entry found in the corrupted system.journal~.
When browsing the journal, journalctl can fall into a loop
where it tries to follow the seqnums, and tries to go the
next location by seqnum, and is transported back in time to
to the older duplicate seqnum. There is not way to find
out the maximum seqnum used in a multiple files, without
actually looking at all of them. But we don't want to do
that because it would be slow, and actually it isn't really
possible, because a file might e.g. be temporarily unaccessible.

Fix the problem by using different seqnum series for user
journals. Using the same seqnum series for rotated journals
is still fine, because we know that nothing will write
to the rotated journal anymore.

Likely related:
https://bugs.freedesktop.org/show_bug.cgi?id=64566
https://bugs.freedesktop.org/show_bug.cgi?id=59856
https://bugs.freedesktop.org/show_bug.cgi?id=64296
https://bugs.archlinux.org/task/35581
https://bugzilla.novell.com/show_bug.cgi?id=817778

Possibly related:
https://bugs.freedesktop.org/show_bug.cgi?id=64293

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