MINOR: stats: add counters for SSL cache lookups and misses

MINOR: stats: add counters for SSL cache lookups and misses

One important aspect of SSL performance tuning is the cache size,
but there's no metric to know whether it's large enough or not. This
commit introduces two counters, one for the cache lookups and another
one for cache misses. These counters are reported on "show info" on
the stats socket. This way, it suffices to see the cache misses
counter constantly grow to know that a larger cache could possibly
help.