[#76453752] Added negative TTL configuration option to Mongo caching sub...

To support negative (null object) TTL added additional configuration configuration parameter ("negativeTtl") in addition to current (positive) "ttl". 

Memcached connector uses appropriate setting in a single cache where as Memory connector maintains two caches (negative and non-negative) with appropriate TTLs since this seemed to be the most straightforward way to add this semantic. 

@greglu, @dongshengbc, @ssimeonov - please, review. There is also second PR for this for s2f.  
But next time, when the same key is requested, Memcached will return null again, right? Then the origin server will be hit again. So, negative cache does not work, is that right? Maybe I missed something.