cert validation cache

cert validation cache

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

To implement the cert validation cache a new template class investigated,
the LruMap which implements a simple lru cache.

The LruMap templete class also used to replace the old Ssl::LocalContextStorage
class which implements a SSL contexts cache.

This is a Measurement Factory project