Fix memory leak detected in usecase tests
Fix memory leak detected in usecase tests

Base & derived class destructor to be of _virtual_ type.
Otherwise, one of the destructos will be invoked in the
following cases:
+ When a derived class object is created and assigned to
  a pointer of base class type, upon deletion of which
  only the base class's destructor will be invoked.
+ When a derived class object is created and assigned to
  a pointer of derived class type, upon deletion of which
  only the derived class's destructor will be invoked.

Change-Id: I57be1fd519c5d40f95dcb5bd28643eebed5528bd
Reviewed-on: http://review.couchbase.org/63826
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

wurikiji/forestdb,wurikiji/forestdb,couchbase/forestdb,hisundar/forestdb,hisundar/forestdb,couchbase/forestdb