journal: limit the number of entries in the cache based on available memory

journal: limit the number of entries in the cache based on available memory

This is far from perfect, but should give mostly reasonable values. My
assumption is that if somebody has a few hundred MB of memory, they are
unlikely to have thousands of processes logging. A hundred would already be a
lot. So let's scale the cache size propritionally to the total memory size,
with clamping on both ends.

The formula gives 64 cache entries for each GB of RAM.