journal: replace contexts hashmap with a plain array

journal: replace contexts hashmap with a plain array

try_context() is such a hot path that the hashmap lookup is expensive.

The number of contexts is small - it is the number of object types.
Using a hashmap is overkill. A plain array will do.

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

real    0m9.445s
user    0m9.228s
sys     0m0.213s

After:
$ time ./journalctl --since=2014-06-01 --until=2014-07-01 > /dev/null
real    0m5.438s
user    0m5.266s
sys     0m0.170s