paranoid_hit_validation directive (#559)

paranoid_hit_validation directive (#559)

This directive controls whether Squid should perform paranoid validation
of cache entry metadata integrity every time a cache entry is hit. The
client transaction experiences a cache miss instead.

This feature does not protect from the corruption that happens while the
hit is being delivered because the validation runs once per client
request when that client request discovers a cache hit. Future versions
may add more checkpoints.

The paranoid hit validation statistics are available by means of several
new cache manager counters, available by mgr:counters, mgr:5min and
mgr:60min requests. The statistics are reported only when
paranoid_hit_validation is enabled.