journal: optimize iteration: skip whole files behind current location

Interleaving of entries from many journal files is expensive. But there
is room for optimization.

We can skip looking into journal files whose entries all lie before the
current iterating location. We can tell if that's the case from looking
at the journal file header. This saves a huge amount of work if one has
many of mostly not interleaved journal files.

On my workstation with 90 journal files in /var/log/journal/ID/
totalling 3.4 GB I get these results:

Before:
$ time ./journalctl --since=2014-06-01 --until=2014-07-01 > /dev/null

real    5m54.258s
user    2m4.263s
sys     3m48.965s

After:
$ time ./journalctl --since=2014-06-01 --until=2014-07-01 > /dev/null

real    0m20.518s
user    0m19.989s
sys     0m0.328s

The high "sys" time in the original was caused by putting more stress on
the mmap-cache than it could handle. With the patch the working set
now consists of fewer mmap windows and mmap-cache is not thrashing.

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