systemd: Backport some journalctl performance improvements

Before:

$ time journalctl > /dev/null

real    6m12.470s
user    5m51.439s
sys     0m19.265s

After:

real    0m40.067s
user    0m37.717s
sys     0m2.383s

Before:

$ time journalctl --since '2015-08-01' _TRANSPORT=kernel

real    1m9.817s
user    0m13.318s
sys     0m56.626s

After:

real    0m0.689s
user    0m0.521s
sys     0m0.221s