19872: Set max_slots lower because mmap uses another FD

19872: Set max_slots lower because mmap uses another FD

- Reduce max slots to 3/8 max fds instead of 1/2 because mmap() uses a
second file descriptor, and we keep the original file descriptor open
for flock()

- Rework how cache slots are allocated to try evicting things _before_
allocating a new cache slot, so the cache should be somewhat better
behaved about staying within its configured limits.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>