[PATCH] cert validation cache

This patch add squid internal cache for cert validator helper. It is
posted as separate patch because investigates the LruMap template class
which can be used to implement object caches inside squid.

The new LruMap template class used to replace old
Ssl::LocalContextStorage class which is used as SSL contexts cache.

Patch description:

This patch add cache to cert validation helper. The following new
options added to "sslcrtvalidator_program" configuration parameter to
control cache behaviour:
 ttl=n TTL in seconds for cached results.The default is 60 secs
 cache=n limit the result cache size. The default value is 2048
 SSL contexts cache.

This is a Measurement Factory project