Expurge long tail content from the cache to keep the number of bans
below this value. 0 disables.

This is a safety net to avoid bad response times due to bans being
tested at lookup time. Setting a cutoff trades response time for cache
efficiency. The recommended value is proportional to
rate(bans_lurker_tests_tested) / n_objects while the ban lurker is
working, which is the number of bans the system can sustain. The
additional latency due to request ban testing is in the order of
ban_cutoff / rate(bans_lurker_tests_tested). For example, for
rate(bans_lurker_tests_tested) = 2M/s and a tolerable latency of
100ms, a good value for ban_cutoff may be 200K.

Merges #2131