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

1 files changed, 2 insertions, 0 deletions

diff --git a/src/journal/mmap-cache.h b/src/journal/mmap-cache.h
index 543a2bf2d0..fe2c83d751 100644
--- a/src/journal/mmap-cache.h
+++ b/src/journal/mmap-cache.h
@@ -25,6 +25,8 @@
 #include <stdbool.h>
 #include <sys/stat.h>
 
+#define MMAP_CACHE_MAX_CONTEXTS 8
+
 typedef struct MMapCache MMapCache;
 
 MMapCache* mmap_cache_new(void);