ScanCache: Observe cache size 0.

If the the cache size is 0 no entry should be cached.
Before the patch one entry would always be cached.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Add new target loadtest.

To show behaviour under load set CACHE_MAX_SIZE = 0, and
run the load test with

make
make loadtest

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>